Andrei Kirilenko, a senior member of the Soviet leadership who was once considered a possible successor to Leonid Brezhnev, was dropped today from a lineup of portraits of Kremlin leaders, signaling the end of his political career.

Kirilenko, 76, who was at the center of Kremlin power for more than 20 years, has been seriously ill for some time and there have been rumors that he would retire at the next meeting of the Soviet Communist Party's Central Committee.

But in the mysteriously ritualistic way in which the Kremlin announces political demise or retirement of its top officials, the portrait of Kirilenko was not placed in the lineup of the top leadership posted along Moscow's main thoroughfares in preparation for celebrations marking the 65th anniversary Sunday of the Bolshevik Revolution.

This suggested that, apart from his illnesses and infirmities, the veteran politician may have been eased out for political reasons.

Pictures of the remaining 12 members of the ruling Politburo were set up in alphabetical order with Lenin, the founder of the Soviet state, in the center flanked by Brezhnev and Yuri Andropov, the former chief of Soviet secret police who is now the number two man in the party.

Kirilenko's departure means that the Soviet Politburo has lost its second- and third-ranking members in terms of seniority this year. Since the death of Mikhail Suslov, the longtime party ideologist, last January, Kirilenko and Brezhnev had been the only remaining people in the Politburo who reached full membership under Nikita Khrushchev.

Although Kirilenko was considered a potential rival of Brezhnev in the mid-1970s, his influence was sharply reduced in recent years and he was practically shunted aside. His age -- he is three months older than Brezhnev--as well as his poor health may have been partly responsible for his eclipse.

The departure of Kirilenko leaves three vacancies in the Politburo. Suslov and the late premier Alexei Kosygin have not been replaced.

Political observers here said one of the younger leaders, Vladimir Dolgikh, 59, was expected to benefit from the latest shift. Both Kirilenko and Dolgikh have been responsible for heavy industry.

During the past year or so, Dolgikh had taken over most of Kirilenko's duties as top industrial manager of the party and was named an alternate member of the Politburo last May.

The departure of Kirilenko and the prospect of new men filling the vacancies take on added significance as observers here are watching intensive jockeying for position between supporters of Andropov and his main rival, Konstantin Chernenko, a protege of Brezhnev.

Kirilenko has not been seen in public since August. Various sources said earlier this year that his ability to work was hampered by a cardiovascular ailment. He also is reported to have suffered a heart attack, but that could not be confirmed.

His portrait was included among the leadership lineup posted for Constitution Day last month. Its absence now, like the disappearance of the portrait of Kosygin two years ago, appears to be a clear indication that he has already retired.

The retirement is expected to be formally announced at the next meeting of the Central Committee on Nov. 15.