BUENOS AIRES, Argentina (AP) _ Christina Onassis, heir to the billion-dollar fortune of Greek shipping tycoon Aristotle Onassis, suffered an apparent heart attack Saturday and died, officials said. She was 37.

Miss Onassis, whose famous father, stormy romances and huge fortune made her an international celebrity, was stricken at a country club swimming pool about 25 miles northwest of Buenos Aires, a doctor said.

Miss Onassis, who was married four times and had a 3-year-old daughter, was the stepdaughter of Jacqueline Kennedy Onassis, widow of the late President John Kennedy who married Aristotle Onassis in 1968.

Dr. Hernan Bunge, a director of the Clinica del Sol Hospital in Buenos Aires, said Miss Onassis was stricken at the weekend home of close friends Marina and Alberto Dodero at the Tortuguitas Country Club.

Miss Onassis fell ill as she prepared to go swimming about 11 a.m. (9 a.m. EST) in a pool at the home. She was taken to a country club first-aid station station for attention, then to the clinic, where she was pronounced dead at 3 p.m., he said.

The body was taken to a morgue for an autopsy and then to a Greek Orthodox church for a ceremony.

The findings of the autopsy were not immediately announced. A police officer at the morgue said a death certificate could not be issued until the cause of death was officially determined.

A woman who identified herself as Annabella Onassis, a cousin, told reporters at the church that Miss Onassis ''died in the bathroom'' of the Dodero home.

''She had been feeling fine,'' Annabella Onassis said. ''In fact, she felt very well during a party yesterday at the club.''

Asked if Miss Onassis' death could have involved suicide, the woman replied: ''A happy person can never commit suicide and Christina was content and happy. She didn't have any (apparent) health problems.''

A woman identifying herself as Mary Onassis, an aunt, said Miss Onassis' remains would be flown to Greece on Monday or Tuesday for burial on the family-owned island of Skorpios beside the graves of her father and brother, Alexander, who died in a 1973 plane crash.

Alexander and Christina were Aristotle Onassis' only children.

Mary Onassis told reporters that her neice had planned to attend a party Saturday night given by the local Greek community.

''Christina was in the best part of her life,'' she added.

The official news agency Telam said Miss Onassis was stricken as she changed to go swimming in the pool at the Dodero home.

Buenos Aires police said the administrator of the country club, site of about 350 weekend homes, told them that Miss Onassis had died of a heart attack. Bunge said Miss Onassis was dead on arrival at the clinic.

A spokesman for the Greek Orthodox Archbishop's office said Miss Onassis' body would remain at the church until the arrival of her fourth husband, French businessman Thierry Roussel, from Paris.

Roussel was expected to take charge of arrangements to fly Miss Onassis' body to Greece for burial on the island of Skorpios, the spokesman said, speaking on condition of anonymity.

Miss Onassis and Roussel were married in 1984 and had a daughter, Athena, in early 1985. Eight months later the heiress filed for divorce, her fourth.

Miss Onassis arrived in Buenos Aires on one of her frequent visits to the Doderos, an Argentine shipping family closely associated with her late father, the legendary Greek shipping magnate whose larger-than-life image is chronicled in books and films.

Aristotle Onassis lived in Argentina as a young man and held dual Greek and Argentine citizenship.

The heiress, who usually stayed with the Doderos at their downtown Buenos Aires apartment or in a luxury hotel, told reporters that she had come to help celebrate the 40th birthday of Marina Dodero.

Miss Onassis inherited her father's shipping and real estate empire when he died in 1975 and was worth an estimated $1 billion.

She reportedly didn't get along with her famous stepmother. After Onassis died, his daughter and other members of the Onassis family reportedly offered Mrs. Kennedy Onassis a multi-million dollar settlement to give up her rights to the Greek magnate's estate, news reports indicated.

She married Roussel in March 1984. Her first marriage was in 1971 to Los Angeles real estate broker Joseph Bolker, who was nearly twice her 20 years. Their marriage ended after nine months. Her second marriage to Alexander Andreadis, the heir of another Greek shipping fortune, also was short-lived and ended in 1976.

Her third, perhaps most widely publicized marriage was in 1978, in Moscow to a Soviet civil servant, Serguei Kauzov. They were divorced less than two years later.

Miss Onassis maintained residences in Paris and St. Moritz, Switzerland. She was born Dec. 11, 1950, in New York City.