The United States plans to hold its first underground nuclear weapons test of the year today in a move that some U.S. officials and members of Congress said could trigger the end of the Soviet Union's eight-month, self-imposed testing moratorium.

A Department of Energy spokesman said yesterday that a "weapons development" test would be conducted at 11 a.m. (EST) at the Nevada Test Site by the Los Alamos National Laboratory. He would not disclose its nature, but another source said it was an early test "for a warhead that would go into the stockpile in the future."

On March 13, Soviet leader Mikhail Gorbachev said his nuclear test moratorium, which began last August and was to end March 31, would continue until the next U.S. test.

One day later, President Reagan, who had rejected the moratorium, sent Gorbachev a letter in which he invited Soviet scientists to observe a nuclear test in Nevada the third week in April. Such a visit, Reagan said, could start a process leading to ratification of a 1974 treaty that limits the size of both sides' tests.

On Capitol Hill yesterday, legislators who had assumed that the United States would not test again until April were surprised by the announcement that a test was to take place today. A hastily assembled group of legislators issued a last-minute appeal to Reagan to cancel the test.

"The Soviets say they will not test unless we do," the legislators wrote in a letter to Reagan. "If the United States now proceeds to test, this will compound the Soviet Union's political gain and our nation's political loss."

Congressional sources said yesterday that the administration had made some changes in its testing schedule because of the Gorbachev statements. One congressman, with contacts in the Defense and Energy departments, said the administration had delayed tests originally set for earlier this year and could have delayed today's test. Instead, he said, the decision was made to proceed because "pressure for the moratorium would build up if they waited for April."

An internal March 12 guide for Energy Department spokesmen said nuclear testing had been delayed "in the last few weeks." Several reasons for the delay could be given, the press spokesmen were told, including "changes in priorities" and rescheduling for programmatic reasons.

The schedule itself was not to be discussed "because it was classified." The last U.S. test occurred on Dec. 28.

Public announcement of today's test -- with the code name Glencoe -- was made in Nevada on Thursday because the expected power of the explosion, between 20 and 150 kilotons, could shake buildings in towns and cities near the test site.

Rep. Thomas J. Downey (D-N.Y.), who has taken the lead in pushing for a comprehensive nuclear test ban, said, "If this test proceeds, it will squander our best hope of containing the proliferation of nuclear weapons. It will squander our best hope of stopping the Russians from upgrading their nuclear warheads to U.S. standards."

In turning down the Soviet moratorium and a call from Gorbachev to resume negotiations on a treaty banning all nuclear tests, Reagan has maintained that as long as nuclear weapons are needed to deter war, testing was required to modernize and maintain confidence in the U.S. stockpile.

The only Republican participating in the news conference was Rep. Jim Leach (R-Iowa), who declared that the United States was "ahead in much more sophisticated" nuclear weapons and that snubbing Gorbachev's proposed moratorium "would jeopardize U.S. security."

Rep. Edward J. Markey (D-Mass.) brought the only light moment to the meeting when he declared that "the U.S. needs another test like Imelda Marcos needs another new pair of shoes."

A White House spokesman said later that the president would "of course take into serious consideration" the congressional views, but there was no intention to hold up the test. He also said the Reagan invitation "was still on the table and this test does not undercut it."

In a related action, Sens. Alan Cranston (D-Calif.) and Mark O. Hatfield (R-Ore.) introduced a measure to cut off funds for all U.S. nuclear warhead tests as long as the Soviets do not test. A similar bill, with 84 cosponsors, has been introduced in the House by Rep. Patricia Schroeder (D-Colo.).

As it stands, however, the fund cutoff may become moot because the Soviets could resume testing before the measure is enacted.

In discussing his proposal, Cranston said, "Talk is cheap, and while the superpowers have been talking over the years, they have continued to build up their arsenals."

Hatfield, chairman of the Senate Appropriations Committee and the subcommittee that oversees nuclear weapons production, also appealed to the White House yesterday to delay the test. Later he said that the administration "not only has rejected their invitation to follow suit, we have indicated through this action that building new weapons takes clear precedence over talking peace."