A longstanding dispute between U.S. and Soviet arms control specialists over how much the Soviets electronically hide important data transmitted by missiles during test flights is once more the subject of discussions between the two superpowers.

Although the two countries have been at odds over this since the late 1970s, American officials expressed fresh concerns to the Standing Consultative Commission, a joint body dealing with questions of compliance with arms control agreements, in meetings that ended in December.

American officials say the issue is expected to be raised again when the commission reconvenes in March, although it has not yet been decided whether there will be a formal American protest or continued pressure on the Soviets to explain their actions.

Over the past 10 years, there have been a number of disputes and allegations about compliance with various provisions of arms agreements. But the United States has never formally charged the Soviet Union with a violation of the 1972 strategic arms limitation treaty, known as SALT I, the 1972 treaty limiting anti-ballistic missile defenses, or the SALT II agreement of 1979, signed by President Carter and President Leonid I. Brezhnev but never approved by the Senate.

The ability of each side to monitor electronic signals, or telemetry, sent back to Earth from missiles being tested is an important tool for verifying compliance with such agreements and to make sure neither side is testing weapons or techniques banned by those agreements.

American officials, especially at the Pentagon, are now concerned that the Soviets encode very large amounts of data routinely from their newest missiles to hide that information from the Americans. These tests reportedly include those last fall of the latest Soviet submarine-launched missile, the SSNX20, and two tests of a new land-based intercontinental missile.

In the SALT II agreement, as in the earlier SALT I accord, both sides agree not to interfere with "national technical means of verification"--a reference to data-gathering satellites and ground radars of both sides used to observe these tests. In SALT II, both sides also agreed not to use "deliberate concealment measures" including "deliberate denial of telemetric information."

But the same SALT II agreement did permit "encryption" of some telemetry. The problem, as one official explained it, is that the agreement does not specify what can be encrypted, or coded, so it can be argued that the interpretation of what is legal is left to each party.

The Reagan administration has cited this ambiguity as a serious flaw in SALT II. It has declined to seek Senate approval of the agreement, but has said it continues to comply with its provisions so long as Moscow does the same.