Federal prosecutors today produced surprise evidence showing a pattern of telephone calls between the secret owners of Marlboro Race Track and the office of Maryland Gov. Marvin Mandel on occasions when the most important decisions about the track were being made.

The day before his friends (now codefendants) purchased the track in 1971, 10 calls were placed from the owners to Mandel's office. That was far above the average of two calls a month placed ruing the 30-month period covered by the records.

Chief prosecutor Barnet D. Skolnik confronted Mandel with the telephone records after eliciting from the governor the statement that he "can't remember any substantial amount of contact" with the track owners around the date of the track's purchase.

After the records were introduced, Mandel insisted again that he "absolutely and positively" did not know his friends owned the track until after he took actions that would have benefited the now defunct half-mile raceway in Upper Marlboro.

The telephone records were designed to bolster Skolnik's contention that Mandel had to know that his friends owned the track at the time he acted. The other important testimony about his knowledge came from Pimlico Race Track owner Nathan Cohen, who said that Mandel met with Hess and others in an Annapolis hotel room and helped devise strategy to obtain passage of a bill adding 58 lucrative racing days to Marlboro's schedule.

Prosecutors have charged that Mandel received about $350,000 worth of benefits from the codefendants W. Dale Hess, Harry W. Rodgers III, William A. Rodgers, Irvin Kovens and Ernest N. Cory Jr. - in exchange for promoting legislation to help the track.

The courtroom came alive today as Skolnik presented the crucial evidence, with defense attorneys rising to object, congregating around the bench and circling the prosecutor as he read the telephone records for an hour in staccatto fashion.

"Sit down, sit down," U.S. District Judge Robert L. Taylor admonished the attorneys. "Stop your speeches. There'll be plenty of time for speeches."

Harry W. Rodgers' lawyer, Thomas C. Green, objected that the records were inadequate because they did not show who called Mandel's office (it could have been a secretary, he said) and whether Mandel was there when the calls were made.

Mandel said he had "no idea" what the calls were about. "You have to realize . . . there are four people in my office, any one of those people could have talked or taken those calls . . ."

The logs showed phone calls between Tidewater Insurance Company, owned by Hess and Rodgers, and Mandel's office on Dec. 30, 1971. The men, using front names to hide the transaction, purchased the track on Dec. 31.

In addition, the records showed calls being made around the following important times:

When the now admitted front man for the owners made a public announcement in January, 1972 that he was the real owner of the track.

When state permission was granted in May, 1972, for Marlboro to transfer its allotted racing days to more profitable tracks.

When initial agreement was reached to merge Marlboro with Bowie Race Course in October, 1972.

When Marlboro stock was transferred in February, 1973, by Hess and Rodgers to Cory, allegedly to help cover up their ownership.

When the state approved the transfer of Marlboro's racing days to Bowie Race Track to help complete the merger in June, 1973.

Earlier today, Skolnik continued quizzing Mandel in an effort to cast doubt on the governor's credibility. The subject was the back-dating by five years of a letter stating that Hess owed Mandel $15,000 in legal fees and the dissolution of Mandel's partnership with Hess in a real estate venture, Security Investment Company. Prosecutors have alleged that Mandel's inclusion in the venture was a bribe from Hess and that the backdated letter was drawn up to make it look legitimate.

"Didn't you tell Mr. Hess you and he had better discontinue the payments, [from Security Investment] because you didn't want to have to show them on any public disclosure statements?" Skolnik asked. He received Mandel's unequivocal "no, sir . . ." in response.

"But didn't you tell Mr. Frank deFilippo [Mandel's press aide] in the middle of 1975 while you were under investigation that you had [done this] . . .?" Skolnik shot back, referring to deFilippo's grand jury testimony that has been kep secret by the prosecution.

"He would have to be wrong," Mandel said of his former press aide and confidant. "Because I don't - never made any such statement."

When Mandel later claimed he had nothing to do with Hess' scheme to back-date the letter, deFilippo again came up.

"Isn't it a fact . . . that you lied to Mr. deFilippo . . . and told him that, in fact, that was a genuine letter actually done by you and Mr. Hess in March of 1968 to show that he owed you legal fees?"

Mandel answered that he had only told his press secretary about an unrelated matter, a preliminary option Hess gave him on a motel venture.

Unlike Tuesday's cross-examination, which was filled with bitter exchanges between Skolnik and Mandel, today the men were almost cool with each other, possibly because of Weiner's constant objections that Skolnik protested were "attempts to coach the witness.

Only once, when Skolnik asked Mandel if his son had laundered a $4,500 payment for him from his partnership in Security Investment did tempers flare. "Absolutely and positively not and you know better, Mr. Skolnik," Mandel answered. "I resent you using that phrase as far as my son is concerned."

Skolnik then produced for the first time two checks worth $4.215 and written by Mandel's son, Gary, and made paybale to cash. Both were endorsed by "Duke's Bar, Benedict D. Duke" and cashed in September, 1973, at the small bar in Leonardtown, Md.

Laughing, Mandel said, yes, he had cashed the checks at Duke's, "Dickie Duke is my nephew." Duke is his nephew by marriage to Jeanne Mandel, whom he wed in August, 1974.

"I had borrowed some money from my son and I repaid him," Mandel said when asked why he endorsed the original check of $4,500 over to Gary Mandel. This entire transaction took place while Mandel was negotiating the divorce settlement that freed him to marry Jeanne Mandel.

Other documents, some already in evidence and others newly presented today, were intended to show how a flurry of phone calls were exchanged about the race track venture while Mandel was vacationing with his friends in the exclusive resort town of Boca Raton, Fla.

A Tidewater phone bill showed three calls to Cory, the track's lawyer from Boca Raton, where Mandel was vacationing with Hess, Harry Rodgers and Kovens.

In Cory's personal calender sheets, the calls were listed as "re r.t.", which Cory's secretary testified earlier meant in relation to race track, Marlboro Race Track.

"You were with these men and they discussed the race track and you knew nothing about it? Skolnik asked rhetorically.

Skolnik referred to Mandel's own desk calender at one point when trying to show that Mandel must have discussed the race track with Hess.

In October 1972, Hess flew to New York and met with representatives or The Bowie racetrack. He then flew to Florida and made a call to the governor's office.

That same day, a station to Mandel's dairy showed "8:00 A.M. Hess." Mandel said he could not read "the shorthand" on the calendar and said he did not know if it meant a breakfast meeting or an appointment of just the reminder that he "wanted to call Mr. Hess."

Skolnik completed his total of 10 1/2 hours of cross-examination of 3:35 p.m. with a long recitation of all the coincidental phone calls to the governor around the important dates of the Marlboro racetrack venture. He asked one more time if the governor knew at the time that his friends bought the racetrack on Dec. 31, 1971.

Glancing for one moment at the ceiling, then returning his gaze to Skolnik, Mandel moved his lips into the microphone and repeated: "Absolutely not, sir."