YOUNG COMMUNIST LEAGUE ARMY BASE, U.S.S.R. (AP) _ Next to a rutted tank trail on this training base between Leningrad and the Finnish border, a poster still exhorts: ''Serve as Lenin Instructed, as the Party Demands.''

The Soviet Union legalized alternative political groups five months ago, but the Communist Party is only just beginning to loosen its dictatorial control over the military.

''At this point, the army does not have a multiparty system,'' said Col. Leonid Aksuita, one of about 80,000 professional political officers among the 4 million military personnel.

Political officers have spent decades enforcing Communist Party orders in the army.

Among the more notable were Nikita Khrushchev and Leonid Brezhnev, who moved back into regular party jobs after World War II service as political officers and eventually ruled the country.

Col. Pavel Medvedev said the party relinquished formal control of military promotions since losing its constitutionally guaranteed monopoly on power in March, but acknowledged there had been no reduction in the party's influence.

Change clearly is under way, however, and political officers are seeking new roles in such fields as psychology and public relations.

This infantry training base, named for the Young Communist League, still drips with red-painted party slogans. Draftees pore over propaganda pamphlets and newspapers in the Lenin Room of their barracks.

A young Azerbaijani with a shaven head hunched over a pamphlet, surrounded by images of the party's revered founder. Just five weeks into training, he already had been assigned as a party activist and said fellow draftees accepted his proselytizing.

Had any soldiers ever tried to organize around an alternative political viewpoint? He shook his head and replied: ''Everybody wants to represent the Communist Party.''

Every unit still has a party committee that concerns itself with everything from combat training to officer housing, soldiers said. At the national party congress in July, reformers tried unsuccessfully to have the committees abolished.

A deputy commander in each unit serves as its political officer.

Party reformers also tried to eliminate that position. In a compromise, the ''zampolit'' officers are turning from political indoctrination to more practical matters like morale, discipline and recreation.

Even Gen. Valentin Varennikov, commander of Soviet land forces and a high- ranking Communist, has a zampolit assigned to him.

''My husband could not be a unit commander if he was not a party member,'' said Lyudmila Leshishina, wife of a major at a paratroop base on the Polish border, near Brest.

Communist control is so pervasive that it will be years before the benefits of multiparty democracy come to the army, said officers and soldiers interviewed during a two-day press tour.

''All the persons I work with are Communists and have the same points of view,'' said Ivan Skrylnik of the Defense Ministry's new information department.

At four bases in three republics, soldiers scoffed at the possibility of a non-Communist political party trying to organize within the army.

''Literally, in the next 10 years, it's impossible, because 98 percent of the officers and sergeants are Communists,'' said Gen. Vladislav Lisovsky, deputy commander of the Leningrad military district.

Gen. Boris Gromov, commander of the Kiev military district and former commander of Soviet forces in Afghanistan, seemed almost puzzled when asked whether the party's influence was diminishing.

''I am a Communist,'' he said. ''I know the decisions of the party and I enforce them myself.''

Switching the political officers from representing the party to representing the government is expected to make little difference immediately, since all zampolits are Communists.

They acknowledged, however, that their lives were changing dramatically.

''People are different than they were three years ago'' and unanimity has given way to political debate, said Col. Pavel Illarionov, a political officer at the Leningrad base.

''We can't give commands anymore,'' said Col. Aksuita, political officer of the Minsk district. ''We have to educate.''

The stereotypical zampolit, who earned the enmity of his peers for interfering with military orders, apparently has not entirely disappeared.

''It's a dangerous question,'' Medvedev replied when asked about such men, then added:

''Their activity has dropped since the Brezhnev days,'' seven years ago. ''It seems as if they are not sure of themselves and not sure of what to do.''

Political officers' duties are expected to change even more because the July party congress decided they must stop preaching the party line and concentrate on the general welfare of soldiers.

''They'll be like, uh, chaplains,'' said Capt. Vadim V. Zvegin, in an earnest attempt to explain.

Many political officers are going back to college to earn degrees in psychology and sociology, said Skrylnik of the military press office, and others are taking the new job of press officer.

''We have much more work with the mass media,'' Illarionov said. ''It used to be they always helped us,'' he added, with evident nostalgia for the days when the press always followed orders.