The primary reason Mikhail Gorbachev seems determined to engage in meaningful arms-control negotiations -- including the signing of a medium-range missile treaty in Washington this week -- is fear of President Reagan's "Star Wars" research program.

He has good reason to be afraid.

The president is bent on pushing his Strategic Defense Initiative through the final hour of his presidency, even if it means bypassing Congress and the 1972 Antiballistic Missile Treaty with the Soviet Union.

In a series of columns, we have revealed Reagan's personal approval of a program, code-named "Zenith Star," to test a chemical laser in space in the early 1990s.

"Zenith Star" was secretly pushed by the president in an extraordinary Oval Office meeting a year ago, on Dec. 17. At that meeting were the president, then Secretary of Defense Caspar W. Weinberger, Defense Secretary Frank C. Carlucci, who was then national security adviser, and the Pentagon's SDI chief, Lt. Gen. James Abrahamson.

The president, we are told, was slightly snookered. He thought he was walking into a simple briefing on different SDI programs. But Weinberger manipulated it into a decision-making session, securing firm presidential approval for various SDI efforts, including "Zenith Star."

What Gorbachev surely knows by now is Reagan admitted at the meeting that the "Zenith Star" test would break the ABM Treaty with the Soviets. That treaty bans the deployment, development and testing of ABM systems based in space.

Abrahamson pushed the president to notify the Soviets of a pending breach of the ABM Treaty, but Reagan wanted to keep the program under wraps -- including keeping it a secret from Congress, which was already making the most of the embarrassing Iran-contra scandal.

On the eve of Gorbachev's arrival for the summit, the president continued his not-so-subtle program to undermine the ABM Treaty. At a speech before the conservative think tank, the Heritage Foundation, Reagan accused the Soviets of being poised to break the treaty with their own antiballistic missile system.

On the same day, Gorbachev told a U.S. television audience that Reagan and his scientists could "indulge" in any SDI research they wanted as long as it did not violate the ABM Treaty.

The two men are likely to spend much of their time this week tip-toeing around that treaty, trying to decide how far either side can go with imaginative interpretation.

Reagan's predecessors -- Nixon, Ford and Carter -- understood that space-based laser weapons would demand a renegotiation of the 1972 ABM Treaty. At least in public, Reagan bought that interpretation early in his administration. When he unveiled Star Wars in 1983, the president signed a directive assuring that SDI was not designed to violate the ABM Treaty.

In 1984, Reagan signed another directive not to transgress the ABM Treaty limits with Star Wars research. But apparently that was before the president realized that his time in the White House would end before SDI became a fait accompli.