The number of homicides in the District declined in 1985, deaths from drug overdoses did not hit the record levels that authorities feared last spring and suicides were down for the third consecutive year, according to preliminary figures for last year.

Health officials said they had no ready explanation for the relatively encouraging trends in the District's violent and self-inflicted deaths. "I'm very, very pleased, but we haven't made any conclusions yet," said D.C. Public Health Commissioner Andrew McBride.

As of late yesterday, there had been 148 homicides in the District in 1985, according to police records. In 1984, there were 175 homicides, according to police spokesman Wendell Samuels.

The number of deaths from drug overdoses, as of Dec. 29, was 140, the same number as in 1984. That number was more than double the total for 1983, when there were 69 overdose deaths among the District's 630,000 residents.

Increased police activity against drug dealers in recent months may have prevented the number of drug overdoses from doubling for a second year, according to McBride. "We had a rash of deaths, several clusters, early last year," he said. "We really worried that we'd be double for the year, but when all the pending cases are counted, we should be about the same."

In March, eight District residents died in one weekend from heroin overdoses after an extraordinarily pure batch of the drug reached Washington streets. Throughout the spring, there were other instances when as many as three people died in one day of heroin overdoses. By May, there had been 71 fatal overdoses, 21 more than at that time in 1984. But the rate slowed during the last six months.

McBride said he and other District authorities are unsure of the reasons behind the fluctuations of the city's violent and self-inflicted death rates. To develop some answers, his department recently signed a contract with Johns Hopkins University to provide studies of the number and probable causes behind these deaths.

The preliminary number of suicides in the District, as of Dec. 30, was 67, according to a spokesman for the Department of Human Services. The number has dropped from 104 in 1976, the decade's highest level. In 1984, there were 70 suicides. In 1983, 86 deaths were classified as suicides. The figures were compiled by the D.C. medical examiner's office.

Spokesman Charles Seigel warned that the statistics may change slightly after final death certificates are filed in a few cases.

The District's experience parallels national suicide rates, which have fallen in the last eight years.

"There seems to be some flattening of the rate," said Dr. Susan Blumenthal, chief of behavioral medicine at the National Institute of Mental Health and former director of its Suicide Research Unit, which was disbanded last month in a departmental reorganization. "It's still increasing for adolescents, but for old people, there's a decrease in the rate of suicide."

According to the National Center for Health Statistics, the suicide rate in the U.S. for 1983, the most recent year compiled, is 12.1 deaths per 100,000 population. For that year, the District rate was 10.8 deaths per 100,000, lower than the 11.5 deaths per 100,000 in Maryland and 12.9 deaths per 100,000 in Virginia.

"We wish we could take credit for the declining suicide rate," said Dr. Gladys Baxley, chief of the D.C. Mental Health Services Administration. "We do know there's more discussion of suicide recently -- people are reaching out and talking about their problems."

Baxley credited a new suicide hot line, which operated for four months on Capitol Hill before closing because of a lack of volunteers, as one factor. She also applauded new suicide barriers being erected on bridges in the city.

Carol Simpson, director of the Samaritans of Washington suicide hot line, said the hot line will reopen this month. She noted that several other local hot lines, as well as a new national youth suicide hot line in Chicago funded by the federal government and advertised by Nancy Reagan, may be helping to prevent suicides.

But Alan Berman, a District psychologist who is the past president of the American Association of Suicidology, said, "Hot lines generally deal with attempters, not completers of suicide. What can make suicide numbers change is the availability of weapons, especially guns."

The association, which accredits suicide crisis centers, also discounts fluctuations of 10 to 20 percent because of variances in the way coroners' offices classify suicides, Berman said.