Sydney Biddle Barrows is a preppie, a descendant of two Mayflower Pilgrims, a slender blond who wears designer clothes, vacations in the Hamptons and contributes to charity. "Very WASP, very straight, very much the puritan," says a former boyfriend.

An entrepreneur, the 32-year-old executive advertised her business in the Yellow Pages, noting "Credit Cards Welcome." The business, according to the Manhattan district attorney's office, was "the largest prostitution ring known to the organized crime control bureau of the police department . . . It grossed more than $1 million a year."

The sensational case of the "Mayflower Madam," as New York's tabloids have dubbed Barrows since she was arrested last month, offers more than a peep into the booming business of high-class prostitution, 1980s style.

When 10 police officers broke down the door of Barrows' Cachet II and Finesse escort services on West 74th Street with a sledgehammer, they confiscated extensive records, including a list of 3,000 clients, many of them business executives in prominent U.S., European and Asian corporations, police said.

The prostitution business "has gone from pimps with red Cadillacs and fedoras to classy women descended from the Mayflower," said Al Goldstein, publisher of Screw magazine, which features 27 pages of call girl ads this month and is displayed on street-corner newsstands. "It's a business, just like McDonald's. It has marketing, health insurance, cost controls and public relations. It's a well-run business."

Lt. William Bayer of the New York police's public morals division estimates that aside from the 12,000 yearly arrests of "traditionalist" hot-pants types who work the streets, there are at least 30 high-class call girl outfits operating in the city, with as many as 30 to 60 women in each. "They advertise freely," he said, adding that police rarely investigate them unless there is a citizen complaint.

Among the more exotic recent cases: Two men pleaded guilty last spring to operating a Lexington Avenue church as a front for a prostitution ring. The Church of Sharing, advertised in the Village Voice, featured naked hostesses, a bar, a buffet, a sauna and two "mat rooms." A Manhattan psychiatrist pleaded guilty to insurance fraud this year for filing phony claims with Blue Cross-Blue Shield for $115-an-hour "sex therapy" sessions at a 34th Street prostitution outfit that called itself Health Management Centers. A Queens man was arrested in 1982 for operating what police said was a 103-city prostitution ring from 14 telephone lines in his Flushing home. After his arrest, police said they answered calls from as far as Australia and Europe to set up appointments with his services advertised as Lucky N Love and Rent-A-Date. The case was later dismissed.

Apart from a brief interchange with a Daily News reporter in which she said she was "a nice girl," Barrows has refused to talk to the press since her arrest. Her attorneys declined to discuss her occupation. Her case will be bound over to the grand jury this month, according to the district attorney's office.

Police say Barrows' escort services, employing as many as 30 women, charged clients between $125 and $400 an hour. Many of the prostitutes were students, models or would-be actresses who turned tricks part-time, police said. Barrows gave them training sessions in etiquette and required regular health check-ups.

"This is one of the biggest cities in the world," said Lt. Bayer. "All the money in the world comes through Manhattan sooner or later. Buildings are going up with $3 million, $6 million, $10 million apartments. If a guy wants to pay $2,000 for a girl -- well, that's like you and me buying a newspaper."

The Mayflower Madam case has touched off a tabloid war between the Daily News and the New York Post, each trying to outdo the other with titillating leaks and sensational headlines. The News was the first to publish 11-year-old nude photographs of Barrows, peddled by Steven Rozansky, her former boyfriend and classmate at the Fashion Institute of Technology.

Last week Barrows filed a $36 million damage suit against Rozansky and was granted a temporary restraining order barring him from selling the photos. In an affidavit, Barrows called Rozansky's conduct "reprehensible, selfish and injurious," and declared that the photos "were taken for my personal use only and in a social context."

"I have no record of convictions and although I am currently under a single charge of promoting prostitution, I believe I will be vindicated," the affidavit stated.

The photos were sold to both the News and the Post. On the day the News published them, the Post declared the nude pictures "not suitable for publication in a family newspaper." The next day, however, the paper reconsidered and published one too. It was accompanied by Barrows' genealogical chart under the headline "A Pilgrim's Progress," tracing her ancestry to Mayflower settlers William Brewster and Thomas Rogers.

A new skirmish was touched off last week when banner headlines in the News proclaimed, "A Prince, 'Madam' and a Wild Party; Saudi bathed women in champagne." It reported that Barrows attended a 1980 party given at the Waldorf Towers by an unnamed Saudi prince, during which the prince bathed a number of hookers in champagne.

The Post, which had bannered "Ivy League Club Trysts" -- alleging that Barrows' escorts had met clients at the Yale club, the Union League Club and other upper-class haunts -- doused the News the next day with a story under the headline "Waldorf: 'Prince's Party News to Us.' The News countered two days later with "The prince's bubbly bath? It washes."

While the Post continues to leak names of alleged clients -- including a couple of well-known athletes and an Arab tycoon -- police are refusing to confirm more details. "This is a big struggle between those two newspapers," said Bayer. "They smell blood -- there's a list of 3,000 names. It's like the Profumo affair -- remember how the British government was brought down?" (The 1963 Profumo scandal, which rocked Prime Minister Harold McMillan's government but did not topple it, involved an affair between Secretary of War John D. Profumo and call girl Christine Keeler, who was also the mistress of a Soviet embassy attache'.

Then he added, "I ain't gonna tell you who's on the list. A lot of innocent people shouldn't get their names smeared. They may have engaged in sex, they may not have. You can't be sure in every case."

Barrows is the daughter of Donald Barrows of Rumson, N.J., and Jeannette Biddle Ballantine Molzer, who is now married to Felix Molzer, a musician and director of the Monmouth Arts Center in New Jersey. A graduate of Stoneleigh-Burnham, a girls' boarding school in Greenfield, Mass., and the Fashion Institute of Technology, she worked as a merchandising trainee at Abraham and Straus before starting her own business.

Elizabeth Collier, a former Columbia journalism school student who posed as a job applicant to Cachet for a school paper last year, remembers Barrows as "really nice, very pleasant . . . very gregarious. She was really well-organized, an MBA type."

Before Collier arrived for her interview, Barrows, who operated under the name of Sheila Devin, told her to "dress like you're going to have lunch with your grandfather at 21," an expensive New York restaurant.

Collier wore pearls and a conservative silk dress, told Barrows she was a graduate student in English at Columbia and filled out an application that asked her to list foreign languages spoken, hobbies and interests. She was invited back for a two-hour training session.

Eight pages of instructions gave tips on how to circumvent police and hotel security and how to dress. "She said most of her clients were wealthy executives that made over $100,000 a year," Collier said. "She made the men sound really nice."

As for the sex, Collier said Barrows told the trainees, "I know you're not going to believe it, but it's really not that bad . . . Really. It's different than when a man makes love to you -- this only takes 10 minutes."

Police began their investigation a few months ago, after a complaint from Barrows' previous landlord on West 80th Street. "You are conducting a 'call-girl service,' " landlord Harold Pine told the Housing Court after suing to evict Barrows last spring. Barrows left the apartment before the case went to trial.

In a gracious stone townhouse on 74th Street off Riverside Drive, the door to Apartment 1B stood slightly ajar one night this week. The entrance was scarred by the blows of a sledgehammer, but inside the one-bedroom apartment, with its pink walls and olive velour sofa and chairs, all was in order.

It was here that police arrested three women, Barrows' associates, and charged them with promoting prostitution. A fourth was arrested the same night by an undercover officer in a $300-a-night room at the Parker-Meridian hotel. Barrows turned herself in several days later, and is now free on $7,500 bail.

The apartment bedroom is lined with white fiberglass desks and file cabinets, now empty. The jacks that served 15 phone lines remain, but the phones are gone. Two empty Dom Perignon bottles sit on the refrigerator. Three velvet and taffeta ballgowns hang in a closet.

One almost expects the woman whom the police called "the most professional madam we've ever come across" to reappear, dressed to meet her grandfather at 21.