Despite earlier American complaints about the practice, the Soviet Union last month again encoded radio signals from a test flight of one of its SS18 intercontinental missiles in an apparent attempt to deny intelligence information to the United States.

News of this incident of what the experts call "encryption of telemetry" was leaked in Washington by persons hostile to the new strategic arms limitation treaty (SALT) with the Soviet Union. One such source said the incident was a serious example of Soviet deceit and American weakness in dealing with the Russians.

Carter administration officials said they also perceived a serious example, but they called it an example of "irresponsible leaking." Last month's incident was not related to a new agreement with the Soviets - to send uncoded radio messages from rocket tests - that is relevant to verification of the SALT agreements, the officals insisted.

The first account of the incident to surface publicly came from John Lofton, a conservation journalist, who asked a question at a White House briefing about a recent SS18 test in which, he said, 29 of 31 radio channels sending information on the test back to earth had been put into code by the Soviets.

Several hours after Lofton asked his question - which White House officials declined to answer - a Senate aide critical of SALT II called a reporter to say there was a good story behind Lofton's query. Later the same aide volunteered some details.

Subsequently George Will, a columnist who has written critically of SALT, referred to the test in a question he asked on "Meet the Press."

According to senior administration officials, the incident occurred in early April, and involved one or two SS18 tests. Different senior officials remembered the number differently.

The test or tests were repeats of similar Soviet experiments last july 29 and Dec. 21, the sources said.

All these tests involved a warhead for the SS18, the Soviet's biggest modern missile that carries just one large thermonuclear weapon. "The Soviets have been having trouble with that warhead," one source said.

Until last July the Soviets had left the complex stream of information radioed back to earth from its rocket tests uncoded, and U.S. intelligence could pick it up with relative ease. In a test flight more than 1,000 channels of radio information are sent back to earth, reporting on the functioning of every aspect of the rocket and warhead during its flight.

In these three SS18 test episodes, however, the Soviets put into code all the information coming from the reentry vehicle, which carries a rocket warhead or warheads back into the earth's atmosphere after its flight through space.

This concerned the U.S. SALT II negotiators, and led to prolonged discussions with the Soviet side. Finally, last month, the Soviets accepted the American position that encoding these radio messages would be prohibited under SALT II if the messages in question relate to a new or modified ballistic missile.

Those who publicly revealed the lastest incident of encoding on a SS18 test suggested that the Soviets were flouting whatever new agreement had been reached. The Senate aide critical of SALT who suggested pursuing the story said the Russians had once again taken advantage of an American government overeager to patch together a SALT agreement.

The administration disagreed sharply. It was said on a high livel that the recent SS18 test came before the two sides had reached an agreement on the encoding issue. Moreover, the United States didn't care whether the test data of an old system like this SS18 was encoded "for the next 10 years," as one official put it.

The important question, officials said, was encoding telemetry from tests of new or modified missiles, since SALT II includes restraints on the introduction of modificiations or new systems. U.S. eavesdropping devices and spy satellites can identify a familiar test of an old system even if some telemetry is encoded, the officials said.

"The Russians accept our position new and modified missiles," one official said. "That's what matters."

The skeptical Senate aide disagreed, and also suggested that the April incident was more significant than portrayed by the administration. The Senator aide questioned whether the United States can be sure a test involved an old missile system if some of the tell-tale information on the test was hidden in a code.