Sydney Biddle Barrows, the "Mayflower Madam," pleaded guilty today to promoting prostitution through a high-class escort service. She paid a $5,000 fine and was set free.

Flashing a "victory" sign and toasting the outcome with champagne, Barrows held a news conference in a Manhattan restaurant after her court appearance and pronounced her guilty plea to a misdemeanor a "mini-pardon." She urged the public to read her forthcoming book and watch a television movie based on her experiences.

Barrows, daughter of a socially prominent family that traces its roots to the Pilgrim ship the Mayflower, ran three escort services that advertised in the Yellow Pages and took credit cards.

Police, who broke down the doors of her office with sledgehammers last October and seized the names of 3,000 clients, said she ran "the largest prostitution ring known to the organized-crime control bureau of the police department . . . . It grossed more than $1 million a year."

However, Barrows, 33, wearing a black-and-white suit with a high slit up the back of the skirt, told reporters today that there was "no way any jury in the United States would have found me guilty." She added, "I've never once met anybody who felt I was doing anything wrong . . . . It's not called the oldest profession in the world for nothing."

Barrows claimed that Manhattan District Attorney Robert M. Morgenthau, who is up for reelection, received "a lot of calls from a lot of very concerned and very prominent people" among her clients. That, she suggested, may have been why the district attorney's office accepted a misdemeanor guilty plea after charging Barrows with a felony, and why she was permitted to keep $150,000 in profits from the escort service.

Barbara Thompson, a spokesman for Morgenthau, said, "We deny that absolutely." She called the plea bargain "appropriate" and typical of the outcome in other prostitution-ring cases. Judges in New York have been reluctant to give jail sentences to promoters of prostitution, often considered a victimless crime.

Laughter erupted in the courtroom today when Judge Brenda Soloff said to Barrows:

"You realize that by pleading guilty you could receive one year in prison, up to a $5,000 fine and a period of three years' prostitution . . . . " Then the judge added, "I mean probation, probation."

Barrows sold the rights to her story for an undisclosed sum to director Robert Halmi for a television movie. Arbor House and Berkeley Books reportedly paid $300,000 for the rights to the book, to be written by William Novak, author of a book about Chrysler Chairman Lee Iacocca and a forthcoming autobiography of House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

The Barrows book, to be published in May, "is a fabulous story," Novak said in a telephone interview from Boston. "Here's somebody involved in the sex-for-hire business, which is normally extremely sleazy, and who has made every effort to do it in a classy and dignified way . . . . It's a mixing of two worlds, the 'Mayflower' and 'the Madam.' "

Novak said he checked with "the O'Neill people" about his work on the Barrows book and "they didn't have any problem with it."

"In large measure, it is a business book," he said. "It's about how the business is run." No prominent clients will be identified, he said. As for tabloid reports that Barrows' clientele included "Arab sheiks and captains of industry," he said "prominent Arabs for sure and, certainly, corporation presidents."

Barrows said today that she pleaded guilty primarily to protect the confidentiality of her clients who, her lawyers said, would have been called to testify in a trial. She said some clients had called after her arrest. "They were pretty nervous," she added.

Asked how she expected people to believe that anyone might pay $1,100 (the fee reportedly charged by some of her women escorts) "for a nice conversation," Barrows replied: "I would never pay $1,100 for a nice conversation, but there are a lot of very wealthy men out there who decided it was worth it."

Would she open another escort service? she was asked. Appearing shocked at the idea, she said, "Are you kidding? . . . . No way would I ever do anything illegal again in my life."