Narcotic overdose deaths in 24 metropolitan areas across the country dropped an estimated 39 percent last year, according to federal estimates, at the same time the availability of heroin declined and its street price rose sharply.

In Washington, there were 18 deaths attributed to heroin, methadone or a combination of the two, a low for the decade, according to Dr. James Luke, the city's medical examiner.

Similar findings have been reported by medical examiners in such disparate areas as Maryland, Dade County, Fla., and Los Angeles County, Calif.

Federal officials attributed the decline in deaths to the joint Mexican-American effort to control the flow of heroin from Mexico to the United States.

"Fatalities in the past have been determined to depend on availability" and strength of the heroin, said Luke.

According to a spokesman for the U.S. Drug Enforcement Administration, "the national average retail purity is now down to 5 percent )pure heroin per quantity being sold) which is the lowest it has been since records have been kept.

"The retail price per milligram is up to $1.69, which is higher than it's ever been," said Conrad Daugherty of DEA. "This is largely attributed to the success of the eradication effort in Mexico a year ago."

According to DEA, 90 percent of the heroin in this country a year ago came from Mexico, while only about 65 percent to 70 percent now is coming from there.

Based on DEA's figures, an average heroin user now would be paying $84.50 a day for his drugs, compared to $64 a year ago.

Heroin "behaves like any other commodity," said Dr. Robert Dupont, director of the National Institute on Drug Abuse. "Heroin use is very much responsive to market trends just like anything else is. There are a lot of people who don't believe that, who think heroin is an inelastic commodity."

Dupont said that when price goes up, use goes down. "And when they use less of it they'll just walk away from their habit."

This is not the first time the annual number of narcotics deaths has declined. Washington, which has pattern similar to that in other areas of the country, had 82 deaths in 1971, 71 deaths the following year, 19 in 1973, 20 in 1974, 34 the next year, 32 in 1976 and then 18 last year.

"The hardest thing to explain is how it (the increased heroin use, which began in the late 1960s) started in the firs tplace," said Dupont, former director of the District of Columbia's Narcotics Treatment Administration.

Dupont said he believes there may have been several factors responsible for the sharp rise in addiction. One was the fact that the children of the baby boom were on their own in society and at a vulnerable age. Another factor was the fact that they already were experimenting with drugs, such as marijuana and LSD, and therefore were willing to experiment with other substances.

The number of narcotics deaths is one of the factors used to estimate addiction, as are the price, the number of persons in addiction treatment programs and the purity of heroin.

The decrease in deaths, and presumably in use, in 1972 and 1973 was related to the interruption of the so-called "French connection," the smuggling of Turkish originated heroin through Marseilles. "There was a sharp decline in the availability of heroin and heroin-related problems," said Dupont, who contends that "crime is cycled with this entire thing."

"The next change in the cycle was the increase in 1974 and 1975. We were very ill-prepared for that. I was around at the time and I was not prepared for the Mexican connection," said Dupont, referring to the switch of the heroin production and smuggling routes from France to Mexico.

The present decline is "clearly related to the Mexican government's spraying of the poppy fields in western Mexico," said Dupont. "I think the level of the problem we've had in this country, the wave character, can best be explained by the level of supply."

Dupont said he had "talked very little about the (present) downturn" because "when the heroin problem got better in 1972 and 1973 there were two reactions to it: one was not to believe it; the other was to think that the problem had gone away."

With an estimated 500,000 adicts in the country, said Dupont, "if we show any signs of reducing our level of concern we again will have a worsening of the problem.