President Reagan's unyielding statements last night on his Strategic Defense Initiative (SDI) seemed to diminish greatly the chances for an arms-control deal with Soviet leader Mikhail Gorbachev at their planned meeting in Geneva.

Reagan, under repeated questioning at his televised news conference, appeared to go further than before in not only embracing "research" for the exotic antimissile plan known as "Star Wars" but in ruling out a deal to limit "testing" and "development" of defensive-weapon technologies.

"This is too important to the world to have us be willing to trade that [testing and development] off for a different number of nuclear missiles when there are already more than enough to blow both countries out of the world," Reagan said.

It is precisely such a deal -- trading limits on defensive weapons for sharp reductions in offensive weapons -- that has been dangled, at least in concept, by Gorbachev and other Soviet figures as the summit approaches.

In an interview with Time magazine just before Labor Day, Gorbachev spoke of a distinction between "research in fundamental science" on space weapons, which Gorbachev said will certainly continue, and "field tests" and "the designing stage," which he proposes to limit through a verifiable agreement with the United States.

Such an agreement on defensive weapons is an essential companion to any deal to limit offensive nuclear weapons, Gorbachev said. "If the present U.S. position on space weapons is its last word," he told Time, the Geneva arms-control negotiations "will make no sense."

Reagan's statements last night came when discussions had begun within the administration about trade-offs at the summit involving limits on strategic defense, a long-term program whose worth probably cannot be conclusively evaluated until the 1990s, if then.

Some administration officials have privately indicated a willingness to explore deals involving limits on Star Wars testing and development, while others, including Defense Secretary Caspar W. Weinberger, have ruled out such bargaining. Last night Reagan lined up with Weinberger, though conceivably he could still modify his position before the Nov. 19 summit.

In theory, the "testing" and "development" of the weapons program are limited by the 1972 Antiballistic Missile (ABM) treaty with the Soviet Union. That treaty commits both nations "not to develop, test or deploy ABM systems or components which are sea-based, air-based, space-based or mobile land-based."

However, the Reagan administration has insisted that it could continue to develop and test technologies that might be part of a strategic defense without violating the ABM treaty. A Pentagon report to Congress in April took a permissive approach to such limitations on legal grounds, arguing, for example, that some SDI components could be tested against "antisatellite interceptors" instead of against incoming missiles to stay within ABM treaty limits. Soviet officials have vociferously rejected such flexible interpretations of the ABM treaty's prohibitions.

Before the president's news conference, the expectation in official quarters was that the Soviets would soon present to the United States -- and then make public -- specific proposals for cuts in offensive nuclear arms in return for U.S. limitations on strategic defense.

Soviet Foreign Minister Eduard Shevardnadze, who is scheduled to meet Reagan and Secretary of State George P. Shultz next week, was reported by Soviet sources last week to be bearing such details. Reagan's remarks last night could affect the timing and tone, if not the substance, of Moscow's proposals.

The Soviets have been told on several occasions, according to a senior U.S. official, that cuts in offensive nuclear arms deep enough to affect U.S. security would have a bearing on the administration's attitude toward and timing of its strategic defense program. Reagan seemed, at least on the rhetorical level, to reject any such trade-off last night.

Despite all that has been said in Moscow, Reagan seemed optimistic that he might be able to persuade Gorbachev that a greater reliance on defensive weapons would be in the interest of both superpowers. This would take a basic change in Soviet attitudes and positions, which is hardly conceivable in the nine weeks before the summit.

The president also spoke of the necessity for the two nations to live in the world together.

Reagan expressed concern that recent statements of Gorbachev could build a euphoria about the summit meeting that could lead people to expect "a near miracle." His own strong backing for SDI last night may do much to reduce the world's expectations for dramatic progress at that meeting.