The House sharply reduced President Reagan's request for research into a "Star Wars" space defense system yesterday, voting to provide $2.5 billion instead of the $3.7 billion he had requested.

The 256-to-150 vote occurred after the House rejected, often by narrower margins, five other proposals: Reagan's request, a GOP fallback position of $2.9 billion and three other Democratic proposals that would have cut funding even more.

The House figure still represents a substantial increase over this year's $1.4 billion. The vote appeared to signal support for the concept, but real questions about how it would work, and wariness about its cost.

The Senate three weeks ago agreed to provide nearly $3 billion for Reagan's space initiative. Differences between the two chambers must be worked out in a conference committee.

The $2.5 billion figure was proposed by the House Armed Services Committee in the 1986 defense authorization that the House has been considering all week. House Armed Services Committee Chairman Les Aspin (D-Wis.) said yesterday the votes showed "a lot of caution [about Star Wars] because it's a new subject."

Reagan's proposal was defeated 315 to 104, with barely half the Republicans supporting it. A Democratic alternative that would have provided $2.1 billion and restricted spending on programs that might jeopardize arms control agreements was rejected 221 to 195, the closest margin of the day.

Reagan launched his Strategic Defense Initiative (SDI), commonly known as Star Wars, in March 1983 as a long-term research effort aimed at using lasers, X-rays, particle beams and other advanced technology to construct a "shield" to protect the United States from incoming nuclear missiles.

The Defense Department has funded laser and similar research for years; since 1983 the administration has requested vastly increased amounts for the research.

For fiscal 1985, Reagan sought $1.8 billion; Congress provided $1.4 billion. He asked for $3.7 billion in fiscal 1986 -- the largest research and development project in his budget request -- and the administration has said it will seek $4.9 billion in the following year.

During nearly a day of debate in the House yesterday, critics of the program, many of them liberal or moderate Democrats, accused the administration of pushing too fast for a program that is little more than an idea.

In addition, they warned that the program could lead to the militarization of space, and said some of the demonstration projects would violate the 1972 Antiballistic Missile Treaty, which prohibits testing and deployment of antiballistic missiles or components in space or the atmosphere.

Supporters of Star Wars acknowledged that the advanced technology it requires is in an experimental phase, but argued that the program is the only hope for getting away from an arms race that relies on the threat of mutual destruction to prevent a nuclear war.

They said that in its research phase, expected to last into the early 1990s, Star Wars would not violate the ABM treaty, and that after that the United States could work to modify the accord.

They also argued that the Soviets are working on a similar program, so the United States cannot afford to fall behind, and that strong support for Star Wars would make the Soviets more willing to reach an arms control agreement at Geneva.

"Not only space but Earth itself can become free of nuclear warfare if SDI research gives us reason to go forward," House Minority Leader Robert H. Michel (R-Ill.) said. "How can we fail to adequately fund such a research program? . . . It doesn't add one nuclear weapon to the stockpile. It doesn't threaten anyone on Earth."

But Rep. Ronald V. Dellums (D-Calif.), who proposed the amendment to cut Star Wars funding next year to $955 million, described the notion of a space shield as "an absurd concept that can never be attained." He maintained that Star Wars would violate the ABM treaty, cause the Soviets to increase their nuclear force greatly and drain the U.S. treasury.

Arms control groups, from SANE to Common Cause, have devoted most of their attention this year to slowing down Star Wars. Lobbyists said the program has become the major weapons battle in Congress, replacing the MX missile, which has dominated the congressional defense debate for nearly a decade.

The MX fight appears to be drawing to a close. The Senate voted several weeks ago to halt MX deployment at 50 missiles, and the House this week cut the number to 40. Differences between the two chambers' bills are to be worked out in conference.

The two chambers must also resolve differences over nerve gas after both agreed for the first time to Reagan's request to end a 16-year moratorium on production of chemical weapons.

The Senate approved $163 million last month for a new generation of nerve gas weapons. The House, which has consistently blocked Reagan's request for renewed production of chemical weapons, reversed course Wednesday and approved $124.5 million for the weapons.

However, it voted to delay release of the money until after Sept. 30, 1987, and then to release it only if the United States had not signed a chemical weapons treaty and NATO allies agreed to store some of the nerve gas.

Lawmakers said Wednesday that NATO is unlikely to go along, but a House-Senate conference is believed likely to drop that provision.

The House's reversal on chemical weapons appeared to have been caused by intense White House lobbying, the desire of some Democrats to take a more pro-defense posture, worries about reports of increased Soviet nerve gas supplies and a sense that the newer generation of chemical weapons will be safer than existing ones.