An undoubted public interest impels inquiry into the way the Central Intelligence Agency deals with defectors. But is exposure on television and in the press the right way to probe the matter?

Almost certainly not. So there are grounds for questioning the revelations by NBC television of the life of Arkady Schevchenko, the former Soviet diplomat who defected from his high post at the United Nations Secretariat six months ago.

Schevchenko is not the first Soviet defector whose treatment by the CIA has acquired notoriety. Only the other day a congressional committee was hearing from a CIA witness the horror story of what happened to Yuri Nosenko, a former officer in the Soviet secret police, or KGB, who defected in 1964.

The agency suspected Nosenko of being a plant with phony information. He was held incommunicado for long periods and subjected to intense pressures. One of his CIA captors, at one point, suggested that he be killed. Relevant information he supposedly had about the assassination of President Kennedy was withheld from the Warren Commission.

In the case of the NBC exposure, the implicit criticism is the reverse. Schevchenko, according to the NBC story, which seemed sound in every detail, lived it up as a defector and acted in a way that was almost certain to attract attention. Among other things, he hired a woman - Judy Chavez, who was the source of the story - for sexual services. He paid her some $40,000, including a trip to the Virgin Islands and money for a car. He lived in a fashionable Washington apartment house.

Taken together, the two stories demonstrate the legitimate public interest on the handling of defectors. There have clearly been abuses. Some kind of discipline needs to be applied, and that means unearthing the facts.

But the two stories also indicate why the fact-finding jobought not to be left to the necessarily capricious workings of journalistic enterprise. In the first place there is a national-security interest.

Defectors provide information valuable to those who make American foreign and defense policy. Breaking the cover of former defectors discourages those who would follow suit. But none of us in the media are in good position to weigh up the loss of information against the value of correcting abuses.

Neither are we well equipped to identify with any clarity the nature of the abuses. Those who focus on the Nosenko case will be persuaded the CIA is excessively secretive and harsh. Those who stress the Schevchenko case will believe the problem is laxity.

Because outsiders necessarily deal on a case-by-case basis, we cannot know the whole story. Inevitably, we feature extremes and are unable to draw the balance.

We are not even well equipped to determine the rights and wrongs of individual cases. Maybe Schevchenko has a story to tell - a story of the trauma of defection and the troubles it brings to family and friends. But he can't tell that story publicly. Maybe the CIA was restrained from controlling his actions by present standards of fair treatment. But the agency can't tell its story in public, either.

Finally, there is the little matter of the First Amendment. Its protection of a free press is now under attack in many different ways. One of the reasons is that some of the leading institutions in the press and television have used the First Amendment as a cover for stories rich in entrepreneurial self-promotion but wholly divorced from the original purposes of the constitutional protection.

The NBC broadcast on Schevchenko seems to me a case in point. It made first-rate television - a bulldog investigative reporter confronting a non-plussed defector with the facts as a CIA agent, whose picture was blacked out, vainly sought to protect his charge.

But the public interest has not been well served. The best NBC can say for itself is that somebody else would have run the story. That seems to me not good enough at a time when the press and television, in the interest of preserving the First Amendment privilege, ought to be taking pains to prove responsibility. And as for settling the serious question of treating defectors, that requires an inquiry by competent authorities working behind closed doors.