Nikolai Ryzhkov, in one of the swiftest ascents to power in Soviet political history, was appointed chairman of the Council of Ministers, or premier, today, replacing Nikolai Tikhonov, who after five years in the office was retired "for health reasons," the official news agency Tass reported.

In one deft stroke, the Presidium of the Supreme Soviet removed Tikhonov, at 80 one of the last holdovers of the old guard, from active power and brought in Ryzhkov, who turns 56 Saturday and is the second youngest man on the ruling Politburo, after Soviet leader Mikhail Gorbachev, 54.

Under Tikhonov, the job of premier was largely a figurehead position. Tikhonov's predecessor, Alexei Kosygin, took an activist role in the job, but after Kosygin retired in 1980, party leader Leonid Brezhnev assumed many of the powers the premier had wielded.

It is apparent that under Gorbachev the premier will become more active. Ryzhkov, with strengths in fields such as economic planning and industrial management that Gorbachev has stressed, is expected to assist actively in the economic rejuvenation the new Soviet leader has championed since coming to office.

"It's Ryzhkov who will be charged with keeping the wheels of government oiled and moving fast," one western analyst here said.

Among the current cast of new, young Politburo members, Ryzhkov has amassed the most experience in industry, management and economic planning. A specialist in heavy industries and economics, he worked as a mining and smelting foreman and engineer for most of his early career.

Like Gorbachev a protege of late Soviet leader Yuri Andropov, Ryzhkov's career bounded forward in 1982, when Andropov came to power and Ryzhkov took over as secretary and chief of the economics department of the powerful 300-member Central Committee.

His rise to the Politburo, along with Egor Ligachev and KGB head Viktor Chebrikov, six weeks after Gorbachev became general secretary of the Communist Party also firmly places him in the new Soviet leader's camp. Gorbachev proposed Ryzhkov for the premier's job, according to Tass.

Ryzhkov was considered just a competent bureaucrat in the Central Committee apparatus until April, when he suddenly became a full member of the Politburo, bypassing the usual step of becoming a candidate member. This signaled to analysts here that he would play a key role in the new generation of Soviet leaders swept into power since Gorbachev took office in March.

Ryzhkov's appointment not only seals day-to-day Kremlin leadership in the hands of men of Gorbachev's generation but also will facilitate the shake-up in the bureacracy and shift to a younger generation in the second and third tiers of government, according to analysts here.

As chairman of the council of 64 Moscow-based ministers, Ryzhkov is expected to assist Gorbachev in bringing in younger ministers and reinvigorating the rigid bureaucratic structures of Soviet government, analysts said. He would also play a key role in the restructuring of ministries that western and Soviet economists have speculated will come with economic change under Gorbachev.

After his appointment, Ryzhkov spoke at the Supreme Soviet meeting in the Kremlin, saying he would devote all his energy and knowledge to justify the high trust of the party and people, Tass reported.

Tikhonov, an ally of former Soviet leader Leonid Brezhnev, became premier in 1980, at the age of 75. A Ukrainian from Kharkov, he, like Ryzhkov, started work as a mine supervisor. He began his government career in the Ministry of Ferrous Metals when Joseph Stalin was still in power.

A Politburo member since 1979, Tikhonov maintains his seat in the ruling body but is expected to resign from it when the Central Committee meets next, which could be within a month.

According to Tass, Andrei Gromyko, who retired as foreign minister and became Soviet president three months ago, read a letter from Tikhonov at today's Supreme Soviet meeting, in which the premier emphasized that doctors had urged him to step down.

"The state of my health has considerably deteriorated lately," Tikhonov said in the letter, according to Tass. "A consultation of doctors persistently raises the question that I terminate my active work and, consequently, retire."

"I want to make special mention of the comradely atmosphere that has been created in the Political Bureau lately," Tass quoted Tikhonov as saying in the letter.

The Presidium of the Supreme Soviet discussed Tikhonov's request, Tass said, and considered his "major contribution to the management of the country's economic and sociocultural development."

The tone and length of the Tass statement of Tikhonov's resignation and the emphasis on his failing health signaled to western analysts here that the old guard figure had been retired with honor.