For millions of music fans, it was no surprise when Jennifer Lopez cracked the charts with the single "Get Right" early this year.

Day after day, across the city, top radio stations played the song, a cut from J.Lo's "Rebirth" CD, released in March.

But now questions are being asked about just how popular J.Lo's song really was. Her record company, Sony BMG, paid to get Lopez radio airtime, according to evidence compiled in state Attorney General Eliot Spitzer's fast-expanding probe of payola.

Besides Lopez, some of the hottest acts in music - Beyoncé, Celine Dion, Jessica Simpson, Audioslave, Train, Franz Ferdinand, Big Mike, Switchfoot, Entercom and Lost Prophets - are entangled in the inquiry.

Investigators have subpoenaed documents from top city radio stations, including Hot 97, WPLJ and Power 105, regarding their relationship with record companies, according to Spitzer's office.

And officials at the nation's biggest radio station chains, including Infinity Broadcasting, Clear Channel Communications, Emmis Communications and Cox Radio Inc., all acknowledge receiving Spitzer's subpoenas.

Payola, the practice of secretly paying for radio play or extra "spins," can manipulate popularity charts and jack up record sales.

So far, Spitzer has uncovered massive direct payments to scores of radio stations and "bribes" to programming executives in the form of vacations, laptop computers and plasma TVs, according to his office.

In other cases, record companies paid for radio contest giveaways in return for spins. "Payola is nationwide and industrywide in scope," said Terryl Brown Clemons, the assistant deputy attorney general who is directing the probe. She said the practice violates federal statutes, FCC regulations and state commercial bribery laws.

Representatives for the radio chains, which dominate what's played in New York and throughout the country, declined to comment.

In New York, Clear Channel owns Power 105, among other stations, and Emmis owns Hot 97 and KISS FM. WPLJ is owned by ABC Radio Networks Inc.

Officials at those stations also declined to comment.

The music industry's four giants - Sony BMG Music Entertainment, Warner Music, EMI Group and Universal Music Group - also have been subpoenaed to provide documentation on their relationships with the stations.

Spokesmen for all four companies declined to comment.

But in July, the first stage of the investigation was completed when Sony agreed to pay $10 million to settle the case and discontinue payola practices.

The probe goes to the heart of the music business and the delicate relationships that can mean the difference between huge profits for record companies and artists or mediocre returns and money-losing anonymity.

Six to 10 spins a day per station on hundreds of stations is the minimum to help make a hit or move a record up on Billboard magazine's closely followed radio play charts, said industry experts.

An automated system used by Billboard tracks how many times a record is played nationwide, according to Geoff Mayfield, Billboard's director of charts.

Increased play and artist exposure provide a one-two punch for the record company. First, it directly affects audience buying habits, said industry experts. Second, radio music programmers at hundreds of stations use the charts to make their selections.

Documents disclosed by the attorney general's office named Beyoncé, Lopez, Dion, Simpson and others as possible beneficiaries of payola.

In one case involving Lopez, in a Jan. 31 memo - just before "Get Right" hit No. 12 on the Billboard Hot 100 chart - an intermediary to Sony BMG music said, "Please be advised that in this week's Jennifer Lopez Top 40 Spin Increase of 236, we bought 63 spins at a cost of $3,600." The same memo detailed that "in this week's Good Charlotte Top 40 Spin Increase of 61, we bought approximately 250 spins at a cost of $17K [thousand]."

Both memos referred to buys on specific stations.

Spokesmen for Lopez, Good Charlotte and other stars declined to comment.

Industry sources said the artists themselves sometimes pay half the payola costs, often without their knowledge.

"Under their contracts with music companies, the artists pay back to the record companies 50% of independent promotion costs, including payola," said an industry source who declined to be identified.

"It's paid out of royalties," said the source. "If the artist doesn't sell enough records to get into a recoup [profit after cost] position, then the record company pays 100% of those costs." Four present and former record companies said payola schemes were devised to help record companies reap returns on substantial investments in the big stars' new releases, including music videos and legitimate production and promotion costs.

Daniel Glass, CEO of New York-based Artemis Records, a small company with a limited promotion budget, said the impact of payola is demonstrated by the scant radio time given independent releases.

"We are 23% to 28% of record sales, but we're 3% to 7% of radio airplay," he said.

"In a way, the industry treated payola regulations like jaywalking laws," said a radio executive and former recording industry boss who requested anonymity. "Everyone knew they were there but they assumed government wasn't watching and they got sloppy. Now they're being enforced." The object of the regulations is to create an honest marketplace where consumers and listeners are not deceived into buying music based on contrived popularity.

Meanwhile, Clemons declined to comment on the specifics of the probe except to say that it is "ongoing and expanding."

However, an industry source said Spitzer's investigators have completed questioning of executives at EMI, have begun questioning executives at Warner Music and won't start querying Universal employees for several weeks.

Sony BMG executive E-mails and other documentation disclosed by Spitzer's office revealed that some radio stations received direct payments according to a spin formula.

It was based on the size of the market, or listening audience, and the number of times the record was played.

For example, in a memo dated June 9, 2004, Sony paid several stations $3,500 for "about 37 spins" of the group Entercom. Larger markets meant bigger payments, as did more spins.

In the Sony probe, two New York City radio industry figures are mentioned in E-mails.

Sony BMG planned to give Michael Saunders, program director of Power 105 - WWPR - a plasma TV and entertainment system in exchange for increased airplay of company releases, according to E-mails.

A spokesman for Spitzer's office said the deal never went through because Sony learned of the investigation.

Saunders did not return phone calls.

In another case, a Sony middleman said Hot 97 deejay Enuff asked for car service to the city from his home in Highland Park, L.I. In return, Enuff was to push a record by Nas & Xzibit.

The memo, dated Oct. 26, 2004, reads: "Worked out a deal with Enuff @ Hot 97 around the Nas & Xzibit record. Enuff asked if he could get him a car from Wednesday - Friday from his house to Hot 97. The amount would be $160.00 + tolls each trip."

Alex Dudley, spokesman for Hot 97, said the station and Enuff, who is still employed there, both declined to comment on that matter.

"No one person at HOT 97 can determine what's played because the music is chosen by a committee and not by the deejays themselves, not Funkmaster Flex or Angie Martinez, and the station does tremendous research," Dudley said.