DURING THE PAST decade Leroy "Nicky" Barnes directed from New York City a drug syndicate that every month dumped millions of dollars' worth of heroin on the streets of New York and other Eastern cities. A great many people - federal drug officials, local police and prosecutors, and ordinary citizens - knew it. But because Mr. Barnes put so many administrative layers between himself and the actual drugs, he continually escaped prosecution. And, in beating a variety of other charges from bribery to murder, he gained the moniker "Mr. Untouchable." Now, at last, Mr. Untouchable has been touched. Convicted under federal law in December of being a major drug trafficker, Nicky Barnes last week was sentenced to life in prison. Federal prosecutors said that Mr. Barnes will never be eligible for parole. Ten members of the Barnes synidcate were convicted with him of lesser charges. All but two were sentenced to from 15 to 30 years in prison. Several were fined heavily. In addition to his life sentence, Mr. Barnes also was fined $125,000.

The conviction of Nicky Barnes places in high relief the recent rapid improvement in the operation of the Drug Enforcement Administration (DEA). Two years ago a congressional subcommittee sharply criticized the agency for wasting too much effort on collaring lower-level drug violators, to boost its arrest statistics, while largely ignoring th major traffickers. That has changed.The number of arrests DEA has made in recent years has dropped sharply. But the agency has increased significantly its arrests of major drug violators, the ones who supply the street pushers. Obviously, striking at the top of a drug network can do more damage than arresting hundreds of pushers. In short, DEA has traded quantity for quality.

For example, DEA undercover agents, working closely with the U.S. Attorney in New York, infiltrated and undermined the Barnes syndicate. The work of DEA agents also has led within the past year to the conviction of a major drug trafficker in Los Angeles (he received a life sentence under the same statute that spelled Barnes's downfall) and to the substantial disruption of two other major drug-trafficking syndicates. DEA officials also have made several bureaucratic reforms, such as revising the criteria for promotion, to spur its agents to go after the harder-to-get major drug violators.

The heroin problem in the United States has eased considerably in recent years. Thanks to the vigorous efforts of Mexican drug officials, the supply of Mexican heroin coming across the border - the major source of illegal heroin for the United States - has been cut back. Along with more rigorous enforcement by local and federal drug officials, this has resulted in a decline in the purity of heroin sold on the street and a lessening of the number of heroin addicts (there now are "only" an estimated 500,000 addicts, down from an estimated 800,000 two years ago). No one should think heroin trafficking is about to be wiped out; prbably someone has already put together a new drug network to fill the gap left by the demise of the Barnes organization. But the import of the conviction of Nicky Barnes is heartening nonetheless: It is that in the future it's going to be much more difficult for someone else to acquire the now-vacant title of "Mr. Untouchable."