IT IS A CALCULATED and crude decision the Kremlin has taken, on the eve of the Belgrade conference, to pick up Los Angeles Times correspondent Robert Toth in an obvious KGB entrapment, to question him on grounds that he collected "secret information . . . over a period of time" and to compel him to remain in Moscow (he was planning to end his three-year assigment on Friday) while the KGB investigation goes on. Obviously, the Soviet authorities wish to use the Toth case to make the point that the United States' Helsinki-blessed demand for freer journalism, and by extension the whole range of Western demands on human rights, is merely a cover for subversion and espionage. The tactic is brazen and if it is continued it cannot fail to chill the East-West air.

Moscow should understand that the responsibility for such a turn would be its and its alone. Not one whiff of evidence had been produced to show that Mr. Toth, an experienced and respected journalist, has been doing anything in Moscow except carrying out his professional duties. He was arrested in the act of receiving a scientific paper on a topic, parapsychology, which Mr. Toth, a former science correspondent, was well equipped to write about and which is regarded nowhere outside the KGB as "secret."

We prefer to think that the Toth affair represents no more than a temporary and perhaps not unanimous Kremlin answer to the familar question of whether to meet foreign human-rights criticism by defiance or compromise.If this is so, then cooler Kremlin heads can be expected to prevail and it will not be necessary for the United States to take other steps to protect from arbitrary treatment its nationals who are journalists.