The entire Soviet Politburo was reinstalled today for new five-year terms in a dramatic confirmation of Leonid Brezhnev's aversion to any change at the rigidly balanced pinnacle of Soviet power that could open the way for a group of younger leaders after he is gone.

Despite serious economic challenges and foreign policy problems that already seem certain to tax the ruling gerontocracy's resourcefulness and stamina to the limit this decade, not a single new face added to the leadership, nor an old one dropped.

From the oldes member of the inner circle, 82-year-old Latvian Communist Party chief Arvid Pelshe, to the newest and youngest, 50-year-old agricultural expert Mikhail Gorbachov, the faces will stay the same through the next five years barring death or disgrace. According to veteran observers, this is the first time since the 1917 revolution brought the party to power that the Communists have emerged from a party congress without making a single change in the leadership roster.

The 14 voting Politburo members now average 69 years of age, and the innermost group of Brezhnev, party secretary Andrei Kirilenko, theoretician Mikhail Suslov, Premier Nikolai Tikhonov and Defense Minister Dmitri Ustinov has an average age of 74.6.

Perhaps nothing underscored so well the rigidty that the lack of change today impiled as Brezhnev's own closing remarks to the eight-day 26th Congress of the Soviet Communist Party.

"Allow me to assure you that the new composition of the party's leading bodies will solicitously guard and strengthen the unity of our party's ranks, which is the earnest of all our achievements," he declared.

For his part, with his closest and longest associates fully reconfirmed in the posts around him, Brezhnev showed he relishes his power and acclaim at the very top. When chants of "Glory! Glory!" erupted from the 5,000 delegates as he revealed that the secret vote of the Central Committee had reappointed him to lead the Soviet Union until 1985, he peered out through rimless glasses at the throng of carefully picked elite workers, farmers, intellectuals, soldiers and scientists who give the party its seemingly mass representation, and his elderly face softened with a smile.

Brezhnev, who turned 74 last December, spoke strongly and emphatically during a 25-minute address to the final session. Bearing the four red Orders of Lenin on his left breast, and three other major awards on his right, Brezhnev looked in good spirits as he read his address, pausing from time to time to take waves of applause from the delegates and hundreds of foreign Communist gathered for the ceremonial close to the gathering, which routinely endorsed party economic and foreign policy initiatives for the 1981-85 period.

"We intend to concentrate all our efforts on two interrelated directions -- communist construction and the consolidation of peace," Brezhnev declared in summary remarks telecast live to the nation. "Our foreign policy is a program of continuing and deepening detente, stopping the arms race." He said the country has "a precise and clear program" toward "lasting, stable peace." This was the only reference he made to his proposal last week for a summit with President Reagan and package of arms limitation proposals. Washington is studying the Brezhnev initiatives, aimed at renewing bilateral "dialogue at all levels" to bring Soviet-American relations out of the deep freeze brought on by the invasion of Afghanistan.

Calling for "absolute fulfillment" of the new five-fear economic plan, Brezhnev also seemed to underline for party cadres thhe impact of the Polish crisis, which is one of the Kremlin's most complex and pressing economic and political problems.

The role of the party as "society's leading force does not come by itself," he admonished. "It is earned and won in continuous, unceasing struggle for the interests of the working people. To consolidate that role, the party is constantly tightening its bonds with the mass of the people, devoting itself to their needs and concerns."

Presiding over his fourth party congress since coming to power in 1964 after Nikita Khrushchev was ousted, Brezhnev emphasized that the goal of an annual economic expansion of about 4.5 percent through 1985 depends on unprecedented higher productivity from the Soviet work force in the face of reduced investment in modernizing industry.

"Nothing comes easily," the Soviet president said, his forehead puckering with the concentrated effort to speak forcefully. "Any improvement of the living standard can be achieved only by hard work on the party of the Soviet people themselves. There is nothing that free and conscious work for one's self and for one's society cannot accomplish."

The Congress, a seminal ideological event that normally occurs every five years, has been normally occurs every five years, has been marked by the blanket uniformity of public views and avoidance of open diasgreement that is the hallmark of the Brezhnev era of collective leadership. Insistence on business as usual despite the advancing age of the leaders and what outsiders say is a sharpening need for new economic initiatives, extended even to the eight alternate (nonvoting) Politburo members, who were all reappointed for five years, and to the 10 powerful party secretaries.

The Central Committee, which functions roughly like an executive parliament for the Politburo, was increased to 319 from 287 named at the 1976 congress, reflecting larger party membership because of steady Soviet population growth. The party accounts for about 6 percent and the 265 million population.

Kremlin spokesmen said the alternate Central Committee membership was raised to 151 from 139, while the Central Auditing Commission, which keeps the party's accounts, and has little direct political power, was reduced to 75 members from 85.

The central bodies now number 545, up 33 from 1976. Forty-two members were not reappointed and 36 others have died or retired since 1976.