President Reagan decided last Friday not to make a new offer to the Soviet Union regarding limits on the space-based Strategic Defense Initiative, setting the stage for his rejection Tuesday night of an arms control deal involving offensive and defensive weapons, senior administration officials said yesterday.

Reagan made clear his opposition to SDI restraints during final instructions to his Geneva arms negotiators as they were leaving last week for a third round of U.S.-Soviet talks, set to begin today.

A proposal for a trade-off of SDI limitations for deep cuts in Soviet offensive nuclear missiles "was discussed," according to an official familiar with the meeting. But it was unclear how seriously this was pressed in the face of Reagan's enthusiastic commitment to his ambitious missile defense plan.

In a nationally televised news conference Tuesday night, Reagan publicly rejected the trade-off, which has been hinted at by Soviet leader Mikhail Gorbachev and suggested by many U.S. experts as a possible path to an arms control breakthrough. Reagan and Gorbachev will meet in Geneva Nov. 19-20.

The president said Tuesday that the missile defense plan is "too important to the world" to swap for a possible Soviet offer of deep reductions in offensive nuclear missiles.

Reagan's rejection of a deal with the Soviets to limit SDI "testing" or "development" suggested to some analysts that he might breach the limits on those activities contained in the 1972 Antiballistic Missile (ABM) Treaty with the Soviet Union.

Administration officials said yesterday this is not what Reagan had in mind. State Department spokesman Bernard Kalb said, "The ABM Treaty clearly permits certain kinds of testing, and allowable testing was what the president was referring to last night. The president has directed that the SDI research program be conducted in full compliance with the ABM Treaty."

An administration report, according to a senior official, projects that the United States can remain within existing ABM test limits -- as the administration interprets them -- until 1989 or 1990. Thereafter, Reagan's successor in the White House may have to decide whether to abandon the treaty or seek amendments if the missile defense plan is moving ahead.

"I think the president never seriously entertained the idea of restrictions on SDI, beyond those which already exist in the ABM Treaty," said Assistant Secretary of Defense Richard N. Perle, widely considered one of the staunchest opponents of any limits on SDI in exchange for deep cuts.

Perle added, however, that "perhaps some people thought Reagan would" consider such restrictions.

Some government officials said they had no doubt that Reagan's decision against a trade-off would carry into the planning for his meeting with Gorbachev. Other officials, though, said they believe it to be "premature" to predict Reagan's summit stance; they suggested that Reagan's thinking could be affected by Soviet proposals in the next several weeks, if these materialize as expected.

The Washington Post reported Sunday that internal discussions of a possible arms trade-off had begun within the administration, but that Reagan's position was unknown and that he appeared to be far from a decision.

Officials said yesterday that the issue was considered at a White House meeting, or meetings, on Friday. But Reagan's views were closely held among a few officials until he was asked about the subject at his Tuesday news conference.

Yesterday, former vice president Walter F. Mondale said the summit meeting probably cannot succeed unless Reagan is willing to use SDI as a "bargaining chip and not a monkey wrench."

In a subdued debate with Reagan's national security affairs adviser, Robert C. McFarlane, in Colorado Springs, Mondale said, "We ought to trade clear restrictions on 'Star Wars' for a clear reduction on offensive missiles."

McFarlane, in his response, did not specifically rule out the use of "Star Wars," as the Strategic Defense Initiative is known, as a bargaining chip -- but he came close.

McFarlane said the proposed space-borne defense system is a more moral approach to national security than existing deterrents because the weapon would be aimed at offensive missiles rather than people.

Perle said Reagan is aware that "it would be very easy to wreck" the missile defense plan by casting doubt on its future.

"The mere suggestion that the SDI program is on the table for negotiation would do irreparable harm to the people and industrial firms working on it" by destroying commitment and motivation, Perle said. He added, "Uncertainty is disaster for a program like this."

The testing of many elements of SDI, Perle said, is covered by the provisions of the ABM Treaty. Those provisions in many cases are subject to legal interpretation. "We are bound and determined not to be more restrictive than the Soviets" in interpreting the restrictions, he said.

A senior State Department official who spoke to reporters late yesterday said the question of what the ABM Treaty permits and forbids is "at the center of the stage" in diplomatic discussions at the present time.

An SDI report to Congress in March set forth U.S. legal justifications for a number of major SDI tests that are planned.

Many of the justifications were highly technical and some hinged on restrictive definitions of what constitutes an ABM "component" or "prototype."