Who will eventually succeed Leonid Brezhnev as leader of the Soviet Union and what would it mean for the world?

These questions, always present in recent years as the health of the Soviet president and Communist Party leader has declined, emerged once again last week with rumors in Europe that he had died. The Soviets have not responded officially to these reports, but in various unofficial ways have made it clear that Brezhnev, 72, while ailing anew, is very much alive.

The succession question bedevils the world because, while much can be inferred about the internal ruling structure of this superpower, very few concrete facts are available to allow any precise prediction about the identity of the man -- or men -- who will succeed Brezhnev.

Since the 1917 revolution, new Soviet leadership has evolved only after intense -- and sometimes fatal -- secret struggles within the Kremlin. Each succession, triggered by death or successful rivalry, has proceeded along its own unique lines, establishing no precedent. Brezhnev is still in place well after his prime in large part because the Kremlin lacks the mechanism for a smooth transition of power.

But during his 15 years as leader, with the aid or acquiesence of party ideologue Mikhail Suslov, Brezhnev has shaped and culled membership of the 13-man ruling Politburo so that his policies will continue through the next leadership cycle.

This is the consensus of East European and Western analysts who have watched Brezhnev's Kremlin since he came to power in mid-October 1964.Brezhnev has shown that a cautious, conservative leadership centered on aging cronies has brought a net gain for the country over Nikita Khrushchev's foreign adventures and Stalin's domestic terrors.

So the new faces will almost certainly be the same old faces: Alexei Kosygin, Andrei Kirilenko, Andrei Gromyko, Dmitri Ustinov and Suslov. And the policies will be familiar as well: detente, confrontation with China, expanded trade for Western technology, repression of internal critics and economic stagnation in favor of strong party control.

It seems unlikely that a single person will succeed Brezhnev as both president and general secretary of the Communist Party. Although as party chief he was de facto chief of state, it was not until 1977 that Brezhnev amassed enough clout to shove then President Nikolai Podgorny aside and assume the mantle of head of the nation.

Kirilenko, 73, a Brezhnev crony who has concerned himself almost exclusively with party matters ever since he was made a Politburo member by Khrushchev in 1962, is generally thought the likeliest candidate for party leader. He stands in for Brezhnev when the leader is sick, and his views, so far as they can be determined from the opaque speeches he and most of his colleagues make, are little different on party matters than Brezhnev's.

Others considered candidates for the post include Brezhnev's white-haired chief of staff, Konstantin Chernenko, 67, an apparatchik who is his boss's closest confidante, and Viktor Grihin, 64, a Moscow party chief who called recently for greater ideological discipline.

From the world's standpoint, the most important aspect of any succession is that a new Kremlin leadership is likely to continue the Brezhnev policy of detente, or relaxation of tensions with the West, with its fundamental ingredient of seeking strategic arms control with the United States.

There is deep Soviet self-interest at work in detente. Arms control makes it easier to compete with America's potent technological sophistication while easing some of the internal economic consequences of the Kremlin's drive for military parity with the United States. Detente with the West also gives the Kremlin a freer hand in dealing with its rival, China.

Aside from Brezhnev and Foreign Minister Gromyko, the man best-known in the West and elsewhere in the world from the Kremlin is Kosygin, 75, the premier who came to power with Brezhnev. He took the lead in foreign policy matters in the late 1960s and met with President Johnson at the 1967 Glassboro, N.J., summit.

Whoever are the successors, it is sure to be Suslov, the Politburo's last direct link to the Stalin dictatorship, who will anoint and legitimize the new leaders. Although scorned by many East European communists as a pragmatist who bends Marxist-Leninist theory to justify Kremlin moves, the gaunt, 76-year-old idcologue is said by younger party men to be the most revered man in the Kremlin.

He is the representative of the Russophile, xenophobic and conservative heart of this country, the man who is said to always favor discipline, tight internal order and stiff party control.