At least four persons in the District and one in Prince George's County died last night of apparent narcotics overdoses, and 15 other victims were admitted to hospitals throughout the District for emergency treatment. D.C. police officials described the deaths in the city as the worst outbreak of its kind in recent memory.

"The probability of it being heroin is extremely high," said D.C. police public information officer Lt. William White. "But we can't say for sure until a report is released by the medical examiner."

Police said the first victims were taken to Greater Southeast Community Hospital about 8:30 last night, and others were admitted to Howard University Hospital, the Medstar Unit of the Washington Hospital Center and D.C. General Hospital.

Homicide and narcotics detectives fanned out across the city late last night and early today seeking information about the source of the narcotics involved and for other possible victims. Police said that during their investigation last night they confiscated a small amount of an unidentified drug and turned it over to the medical examiner's office for analysis.

At a hastily arranged news conference at 7th District police headquarters, Assistant Police Chief Isaac Fulwood asked that people refrain from using any kind of narcotics, warning that police had not determined the source and cause of last night's outbreak.

Police officials said that one large supply of narcotics probably was responsible for last night's deaths and illnesses. They noted that the purity of heroin available in the District had increased significantly over the last three years.

"I would imagine that it's one package of heroin," homicide Detective Russell Drummond said. "Every now and then, the drugs are so potent that junkies just cannot handle it. It's rare, but it can happen."

Police said that although the overdoses ranged throughout the city, most of the purchases appeared to have been made in a small area in the Stanton Road neighborhood of Southeast.

While Drummond and other officers speculated that all the narcotics came from a single cache, they added that it was not certain whether the drugs involved were tainted or unusually potent.

Four of the surviving victims were reported to be in critical condition last night, according to doctors and police officials.

Drummond said that the toll "could go higher . . . . They are scattered across the city."

Officials at District hospitals confirmed the numbers and severity of the drug-related emergencies. "This is as bad as I have ever seen it," said a nursing supervisor at Greater Southeast.

An attendant at the D.C. medical examiner's office said he had dispatched a vehicle to pick up the dead, but that toxicological reports conclusively establishing the cause of death would not be available until today at the earliest.

But police officials were quick to blame the deaths on the increasing purity of narcotics in the area and on lower prices.

"I cannot remember a time when heroin was more abundant in this city," said White. "You do not have to work very hard to find it or afford it."

The quality of narcotics that has been entering the United States has improved markedly in each of the last three years, police officials said.

No arrests were made immediately in last night's outbreak, and police said that they did not have any suspects. They said, however, that they were working on the assumption that the drugs came from a single supplier.

The names of three of the four persons who died in the District in last night's outbreak were released by D.C. police early this morning. Two of them were District residents, one from Southeast and one from Northwest. The third was a resident of Prince George's County.

They were listed as Arthur Keeling, 28, 4553 Akron St., Temple Hills; Harold Ferguson, 29, 845 Xenia St. SE, and Henry Edwards, 34, 1631 Euclid St. NW. The name of the fourth victim was withheld pending notification of relatives, police said.

A fifth man was found dead of an apparent overdose on Walker Mill Road in District Heights in Prince George's County. D.C. police said they were not certain if his death was connected to the outbreak.

The name of the fifth man was not immediately available.

Working from the 7th District station house, White called the outbreak "extremely troubling and significant."

Homicide detectives could not say immediately how many drug-related deaths have occurred so far this year in the District, but there has been a steady rise in mortality figures since 1979, they said.

In 1981, 115 persons died in the District of drug overdoses, one of the highest figures reported until then. But that total was exceeded last year, police said, and the rate so far this year indicates that the 1985 totals might surpass those figures.

Last summer the District experienced a surge in drug-related deaths. In August alone, 24 persons died of appearent heroin overdoes, according to police and hospital authorities.

At the time police said that a lethal batch of heroin had come into the country and that prices were at a record low.

All jurisdictions around the District experienced an increase in drug-related fatalities last year, police said.

"But this is the worst yet," said White. "And I don't see anything out there that is keeping it out of the country."

Narcotics detectives said the concentration of pure heroin in most drugs sold on the street is about two or three per cent. Last year a batch twice as strong, about 6 percent, was believed responsible for several deaths in the District. They said they did not yet know the concentration of the drugs involved last night.

In what appeared to be the most recent similar incident, four persons died of apparent overdoses of heroin in less than 24 hours in early November 1982. Four others died that month.