Alexandria police have charged a former city resident with murder in connection with the 1984 slaying of prominent pediatrician Robert S. Rixse, who was shot several times at his front door while his three daughters slept upstairs, according to sources here and in Florida.

John Kennedy Robinson, 51, who married Rixse's wife, Diane, after the Rixses' bitter divorce and custody battle in 1981, is being held in Florida after failing to make bond on an apparently unrelated federal drug charge. He has been charged with murder by the Alexandria police, an official at the Escambia County Jail in Pensacola said yesterday.

Police officials here declined comment, but sources said the murder charge followed a recent break for investigators in their attempts to solve the three-year-old slaying, which police had long suspected was executed by a hit man.

Rixse, a popular Alexandria physician and civic activist, divorced his wife on grounds of adultery and won custody of their three daughters. The Robinsons, with the Rixses' children, moved to Marietta, Ga., not long after the slaying and have since divorced.

At the time of the shooting, the Robinsons were living in the Beverley Hills section of Alexandria, a five-minute drive from the doctor's home at 315 Kentucky Ave. The slaying of the 37-year-old physician occurred amid a protracted court battle over division of the Rixses' property.

Shortly after the killing, Robinson told The Washington Post that he and his wife were "totally baffled" by the murder. "We had absolutely nothing whatsoever to do with that man's death," he said. "It was an absolute surprise to me. I was absolutely flabbergasted."

John Rixse, Robert Rixse's brother and an employe of the Central Intelligence Agency, said yesterday, "I had every confidence that the Alexandria police would bring to justice the person or persons responsible for my brother's death. It sounds as if they are successfully moving in that direction."

Rixse, who was born in the District and raised in Alexandria, was shot at close range with a small-caliber handgun when he answered persistent knocking at his front door at 11:20 p.m. July 1, 1984. Clad in cut-off blue jeans and a bathrobe, he was upstairs writing checks for monthly bills when his slayer came to the door, leaving a car idling in the driveway, said police.

Rixse's three daughters, then aged 11, 10 and 7, were asleep upstairs at the time of the slaying.

Neighbors said they saw a dark-colored compact car drive slowly away from the home with its headlights off after the shots were fired.

Police detectives have long thought that the killing was done by a professional hit man and early on ruled out mistaken identity or burglary as motives. Though they believed they knew who was responsible for the slaying, they were unable to bring charges for lack of evidence, they have said.

As part of their investigation, police detectives attempted to interview Gerald Lyle Hemp, 48, a convicted drug dealer whom Robinson once knew, police officials here have said.

But they were thwarted when Hemp, serving a 40-year sentence for a 1983 Tennessee drug-smuggling conviction, escaped from a minimum-security work-release center in Florida 10 days after Rixse's slaying.

Detectives have said interviewing Hemp would have been "just one step" in the Rixse investigation. According to an informed source, police believe Robinson contacted Hemp for help in locating a gunman to kill Rixse.

Last November, Robinson, Hemp and three other men were indicted by a federal grand jury in Tallahassee, Fla., on four drug counts stemming from activities that occurred between 1980 and 1982, according to federal prosecutor Alan Sprowes in Tallahassee.

Robinson was arrested Aug. 5 in Marietta on the drug charges and extradited to Florida Aug. 14, federal officials said. He was unable to post a $25,000 bond for those charges, Sprowes said.

Meanwhile, Alexandria police obtained new information about the Rixse slaying. Acting on this, a search warrant was executed last month on Robinson's office in Marietta, according to sources.

Among the documents seized was an account by Robinson of how the Rixse slaying was carried out, according to several sources. Alexandria police then filed murder charges against Robinson. They will be seeking his removal to here to face the murder charges, sources said yesterday.

Police officials said yesterday the Rixse investigation is continuing.

Diane Rixse Robinson could not be reached yesterday for comment. In late 1984, she complained that the Alexandria police were using "Gestapo techniques" against her in their investigation.

A police official at the time said Diane Robinson and her husband had been "extremely uncooperative and defensive" in the investigation.

In his deposition in the 1981 Rixse divorce case, Robinson described himself as a graduate of Mercer University in Macon, Ga., a member of the Georgia bar, a member of the D.C. bar, and a graduate of the Harvard Business School.

None of the statements is true, according to letters from the institutions, which Rixse solicited and submitted as evidence in the proceedings.

Rixse's civic activities and involvement in school sports had made him a popular figure in Alexandria, where residents were shocked by his slaying. A new swimming pool at T.C. Williams High School was named for Rixse.Staff writer Philip Smith contributed to this report.