Soviet leader Leonid Brezhnev, whose absence from official functions this week sparked intense speculation yesterday that he was gravely ill, was said by some Soviet sources today to be sick but not seriously so.

The sources did not specify the illness of the 72-year-old president and party chief. But they said there was no basis for the rumors that swept Western captitals and stock exchanges yesterday that he is in critical condition, or had died.

Soviet media made no mention today of his health, and Andrei Kirilenko, one of his closest Politburo cronies and a likely major figure in any succession crisis, was still reported on an official visit to Hungary, ending Saturday. There were scattered reports from Western and credible Soviet sources that Brezhnev had been seen on his way to and from the Kremlin earlier this week.

Brezhnev, who will be 73 in December, has been in uneven health for some years. His absences from official functions have invariably caused rumors of serious illness. The phenomenon underscores the paucity of reliable information available about the 13 Politburo members, whose average age is near 70.

On Monday, Brezhnev failed to show up for the visit of Syrian President Hafez Assad and Syrian sources in Damascus reportedly said they had been told in advance tthat Brezhnev was ailing and unable to greet their leader.

A Syrian-Soviet communique issued today indicated Moscow has agreed to new arms aid for Syria.

Although the list of suspicions is long, there has never been a reliable public account of what serious illnesses afflict the Soviet leader. Over the years, he has been said to have suffered strokes or heart attacks, to have a form of lymphatic cancer, or emphysema, or amytrophic lateral sclerosis, also known as "Lou Gerig's disease." His shuffling gait, slurred speech, puffed face, dulled manual dexterity, and short energy span have all fueled these theories.

He wears a hearing aid and is accompanied in all public appearances by military and civilian aides who help support and guide him.

Brezhnev last was seen publicly Oct. 8 on a state visit to East Germany. He has traveled widely this year to Siberia, the Ukraine, and Bulgaria, as well as to Vienna in June for the summit with President Carter. Vienna was agreed to because Brezhnev's flagging vitality apparently ruled out a long flight to Washington.

When Brezhnev disappeared last spring for several weeks, reports circulated here that the Politburo had agreed to an emergency succession plan if he became incapacitated. Under it, Premier Alexei Kosygin, 75, widely experienced in foreign affairs, would have become interim president. Kirilenko, an experienced party apparatchik, reportedly would have become interim party leader.

Most Western analysts tout the two men as the most likely current candidates to succeed Brezhnev. During his 15 years in power, Brezhnev has effectively weeded out younger potential rivals and bolstered the influence and power of the "Dnieper Mafia" of lifelong party cronies from their years together in the Ukraine. The youngest member of this group is Konstantin Chernenko, Brezhnev's personal chief of staff, who is 67.

Standing behind Brezhnev's steady accretion of power is Mikhail Suslov, 79-year-old party ideologue, who would function as kingmaker in any succession. It is widely thought here that he was the Politburo member treated by three Johns Hopkins specialists this week.