The nation's military reserves, now being activated, are an untested force whose performance in coming months will help determine the kind of armed forces the United States will build for a post-Cold War world, defense manpower specialists said yesterday.

In the last two decades the Pentagon has restructured forces to put more reliance on reserve and National Guard units to flesh out the standing military in case of major mobilizations.

"It's a real test for the whole future of the reserves," said Martin Binkin, a senior fellow at the Brookings Institution who has specialized in military manpower issues. How the reserve units perform will go a long way toward answering "whether this unprecedented dependence on reserve forces is a reasonable option for protecting our national security."

Active-duty military leaders, especially those in the Army and Navy, have long been skeptical of the ability of part-time warriors to perform adequately when they are sent to the battlefield or in a support role behind it. They point to National Guard outfits that fought poorly in Korea and Army reservists who sued the government rather than go to Vietnam.

"Our service is literally choking on our reserve components," Maj. Gen. Robert E. Wagner wrote Gen. Carl E. Vuono, now Army chief of staff, decrying by investing so much of the military budget in the National Guard -- run by state governors in peacetime -- and Army reserve units, which train about 38 days a year.

"Our reserve components are not combat-ready, particularly National Guard combat units," Wagner complained in his 1986 letter to Vuono, then head of the Training and Doctrine Command.

"Roundout is not working," Wagner, then commander of the Army ROTC Cadet Command, said in assailing the technique of making one of an Army division's three brigades a reserve outfit. "These forces will not be prepared to go to war in synchronization with their affiliated active-duty formations."

Wagner is but one of many who challenge the so-called total force concept to put so much military capability in the reserves that active-duty forces cannot go to war without them.

Retired general Bernard W. Rogers, former European commander; the General Accounting Office, and Congressional Budget Office are among others who have warned that shortcomings in reserve units make it questionable whether front-line forces could stay in the fight.

Congress over the last decade has insisted that the Pentagon invest heavily in reserve forces, earmarking billions of dollars to purchase first-line planes, tanks and guns for the National Guard, for example. The prevailing argument has been that reserves provide the most bang for the buck if they are equipped with modern weaponry.

Before the United States ended the draft in 1973 -- switching to an all-volunteer Army, Navy, Air Force and Marine Corps -- the president could send a strong signal to the world by mobilizing the reserves. To achieve the same effect on the civilian sector today, manpower experts said, President Bush would have to reactivate the draft, a step he is not expected to take.

"We always seem to be fighting the last war," said Lawrence J. Korb, former Pentagon manpower chief, in agreeing that Bush's call-up of reservists will not have the political impact of past mobilizations, when the draft was in effect.

The Pentagon has not identified which units will contain the 46,703 reservists, who will be activated as units, not individually. Most are expected to play a supporting role, instead of conducting front-line patrols on the Saudi Arabian desert. Doctors, lawyers in civil affairs units, cargo handlers, truck drivers, mechanics and technicians who know how to run water desalinization plants are among the most sought-after reservists, defense officials said.

Part of the Pentagon's war plan for the Persian Gulf calls for the Army's 24th Infantry Division (Mechanized) to go to the trouble zone with two active-duty brigades and a third "roundout" brigade of reservists. But rather than rely on the reservists, many Army leaders have pushed for rounding out the 24th Division with a brigade from the 101st Airborne Division, according to Pentagon officials.

"This is a vote of no-confidence in the roundout concept," Binkin said. If reserves are not going to be used for front-line duty in this call-up and the Army no longer has to dedicate existing active-duty divisions to the Soviet threat to Europe, he contended, the restructuring of the military for the post-Cold War period might be best accomplished through shifts within the active-duty force.

This would mean a reduced role for reserves in the next century, a trend Congress almost certainly will fight for philosophical and economic reasons. The guard and reserves approximate the long-cherished concept of citizen-soldier and are far cheaper to maintain during peacetime.

If past reserve call-ups are a guide, Bush might encounter such problems as: reservists and their families going on television to complain that the wage-earner was forced to leave home only to sit idle at a military base; reservists going to court to contend their call-up was illegal, or that they were not trained for their jobs.

During the Vietnam War, for example, 91 reservists at Fort Meade filed a suit declaring that they were too poorly trained to serve in Vietnam. That suit went to the Supreme Court, before it was rejected.