YOU MAY THINK that if you jail a drug dealer, he can't deal drugs. Not so.

You may think that a 70-year sentence, certainly a life term, should slow a man's step and secure society some sensible respite from his crimes. Actually, the more drugs the dealer distributed and deaths he dealt, the shorter his prison stay may be. Who puts such a dealer in striking distance of the street? The very one who put him in prison -- the prosecutor.

This may bother you. As a former prosecutor of drug dealers it makes me furious. I would expect a narcotics dealer facing years in prison to seek a deal with the government that would get him out. But I expect better of the government than for it to repay the most vicious offenders with a relatively comfortable life and the possibility of freedom.

The government cannot ignore the effect on youths when a pusher is found guilty, but still succeeds by dealing again -- not drugs this time, but information.

Consider what happened to two dealers.

Leroy "Nicky" Barnes of Harlem was poor and addicted to heroin. He kicked the habit only to hawk narcotics instead. He sold them so well that he was one of the top 10 drug traffickers in New York by 1970. He was, however, hard to catch. He was arrested for murder, narcotics offenses, bribery and illegal possession of a gun leading to the discovery of $130,000 in a suitcase in his car; he was acquitted in every case. So they called Nicky Barnes "Mr. Untouchable."

In 1972, Barnes formed "the council" to manage and supervise the distribution of narcotics in Manhattan, the Bronx and Brooklyn. The council shielded Barnes further from detection. By 1975, Barnes declared an income of $288,750. He said $1,750 of that was wages, the rest "miscellaneous income." When he celebrated his 44th birthday in October 1976, more than 200 alleged members of organized crime attended.

In March 1977, the federal grand jury in Manhattan indicted Barnes and his top lieutenants for distributing 44 pounds of heroin a month. I prosecuted one of Barnes' alleged suppliers. At his arraignment he stood quietly, glaring, with his arms drawn tightly across his chest. He said only two words, "Not guilty."

President Carter considered the case so significant that U.S. Attorney Robert Fiske himself tried Barnes. In his opening remarks to the jury, Fiske said Barnes "made the big decisions and the big money" and told them not to be surprised to learn that Barnes was "not on the street selling heroin any more than the president of General Motors sells cars on the floor of showrooms." After the two month trial, the jury convicted 11 of the 14 defendants including Barnes. Touch,e! No more untouchable.

The investigation and two-month trial cost the taxpayers an estimated half million dollars but the New York regional head of the Drug Enforcement Administration (DEA) said, "Whatever the cost to the government, it's miniscule compared to the misery Barnes has brought to addicts and their families."

At Barnes' sentencing in January 1978, Fiske charged that Barnes ran "the most venal drug ring in New York City." The judge said that Barnes was "a great danger" to the community and sentenced him to life.

Law enforcement officials were jubilant. The regional head of DEA said that his agents "risk their lives and see the dealers walk in and out of courtrooms as if they owned the doors," but "this time justice triumphed." A drug rehabilitation counselor said "No one on the street believed the criminal justice system could convict Nick Barnes and send him away for life."

The "street" may yet be right.

Sterling Johnson, the city's special prosecutor, said that law enforcement officials suspect Barnes might have tried to control his former drug operation from his federal jail cell. Johnson said that Barnes' alleged lieutenant, Guy Fisher, who had been indicted but acquitted, and another alleged lieutenant, Frank James, "might have been a holding operation for awhile." Instead those two apparently "decided to go (into business) on their own."

While Barnes was stewing in prison, he also learned that two women were "unfaithful" to him. Barnes testified later that they had "run around with members of the council, which was the only thing I asked them not to do."

Feeling wronged by friends and associates, the former untouchable agreed to do the unthinkable -- give evidence against them. Since then, Fisher, James and both women have been indicted. Barnes not only told the government about the full extent of his drug dealing but also revealed how, as a member of the "council," he voted to "terminate" (kill) six men regarded as informers and robbers. He said he did not "pull the trigger." He only conspired to kill. So Barnes struck a bargain with the U.S. attorney, who by then was no longer Fiske, the original prosecutor, but Fiske's successor, U.S. Attorney John Martin.

Last November, Martin took the unprecedented step of confirming that Barnes was cooperating. Martin's deal with the dealer was that we taxpayers would protect Barnes in prison. Isn't that ironic? It means private cells and other comforts for Barnes.

The deal also says the government will protect Barnes if he gets out of prison. Barnes' sentence is life without parole. Before this deal, ''getting out" was an impossibility. Now, Martin has said that Barnes' cooperation has been "extremely helpful" and that he would not to object to a presidential pardon for Barnes. Government officials may wink reassuringly that Barnes will never get a pardon, implying that all Barnes gets out of this is revenge against his former subordinates and a more comfortable prison life for himself.

The winking reassurance may be as substantial, however, as the smile of the Cheshire cat. Barnes' former lawyer said that until this agreement, "there was no light at the end of the tunnel. Now there is." Asked directly if his cooperation would result in his freedom, Barnes testified, "I would hope that it would, yes." As for Martin, he resigned recently, to be replaced by still another U.S. attorney.

Barnes is in jail. Frank Lucas used to be.

Frank Lucas, born to rural North Carolina poverty, left it for Newark's central ward as soon as he could. He too sold heroin. His "council" was known as the "country boys." He had no major problems until January 1975 when he was indicted on federal drug charges. Federal officials seized half a million dollars from his New Jersey home during his arrest. Following his conviction, Lucas was sentenced to 40 years.

In June 1975, Lucas was indicted a second time by a federal grand jury. Things looked bad. The trial ran for 6 weeks. On Christmas eve 1975, the tension broke. Defendants and lawyers in the courtroom laughed, applauded, and shouted, "Merry Christmas." Lucas had been acquitted.

The joy of the season was short-lived. Before Christmas 1976, Lucas had been convicted again of narcotics trafficking and sentenced to 30 years. This time the sentence was imposed, not in federal court, but by Superior Judge Ralph Fusco of the state of New Jersey. Judge Fusco sentenced Lucas, then 46 years old, to a 25-to-30-year prison term, to start only after Lucas served his 40-year federal sentence. Judge Fusco said he intended this combined 70 year sentence to keep Lucas "off the streets." For life.

Even in jail, Lucas would not stay out of trouble. In April 1977, he was named in a 21- page indictment for conspiring while jailed (by using coded instructions to his visitors) to possess and distribute heroin in Queens, Manhattan and Bronx Counties. Barnes and his council, you may recall, had their own corrupt designs for Manhattan and Bronx Counties. It is not surprising, therefore, that the indictment charged that Lucas "discussed killing Leroy Barnes . . . in order to eliminate heroin competition to the Lucas brothers' organization."

With this last indictment, it appeared impossible for Lucas ever to get out of jail alive. That changed in July 1977 when he pled guilty, admitting that he ran a multimillion- dollar drug ring from prison. Lucas finally agreed to cooperate, to testify. There were the same reassuring winks.

In 1981, Lucas' 40-year federal prison term was suspended. Last summer, at a closed hearing in a New Jersey courtroom, his 30- year state sentence was reduced to the time he served in federal custody. Exactly what the government told the court isn't known because the records are sealed. What is known is that Lucas had the last laugh: he hit the street after serving seven years of his 70-year sentence. Will Barnes be able to top that? In 1985, it will be seven years after he was sentenced.

Why should the government allow this? Why should it permit a criminal such as Barnes to hurl his former subordinates to the prosecutors to feed the government's appetite for arrest statistics? The government cannot overestimate the harm these cases do to the morale of agents who literally risk their lives putting these criminals away.

The government should be prepared to say that there are some people -- and in my opinion Barnes is one of them -- that shall remain as untouchable after conviction as they made themselves before. As a prosecutor, I told the sentencing judge in a major narcotics prosecution that under no circumstances would the government even consider an offer by that narcotics violator to cooperate. The government did not and he is serving his 20-year sentence. The government should do that more often.

I think sometimes the government loses sight of the hurt that heroin causes, and of the human cost. I don't know if you have ever seen it. I have.

Late on a lower East Side street years ago, a man stood in a cone of lamp light 15 feet ahead with his back to me as I approached. He stopped to pick up something -- so I thought -- but he didn't touch the pavement. He did not appear to move at all as I stepped, then stepped again. The only sound on that lonely street were my steps. Until, in what seemed like a heart beat, he spun in place to face me, standing straight the instant he turned. Air came hissing from his mouth. His watery eyes seemed afloat, glinting in the light. They widened and looked past me -- they did not see -- and stared straight upward, reflecting the street globe's glow. His back arched and his left arm swung into the lamplight, a needle stuck in it. The needle should have fallen but it didn't. Then he looked at me. His grin was friendly. He was -- there is no other word -- in rapture. For the moment.

I saw this heroin addict with a "rush" in his moment. Today, he is probably dead. That is what this is all about.