CHICAGO, NOV. 7 -- Rep. Kenneth J. Gray, a 12-term Democrat, said today he will not seek reelection next year because of health problems and political opposition.

"It is with a great degree of sadness that I announce today that I will be unable to seek a 13th term in the U.S. Congress," Gray said in a statement.

"Once again, the Republican National Congressional Campaign Committee has targeted the 22nd District as one that will receive extra funds {and} numerous opposition speakers," said Gray, 62.

He said the GOP plans "an all-out 12-month campaign between the filing time in December 1987 and the November 1988 general election."

Gray said he suffers from a leg muscle ailment caused by a tick bite he received in the jungles of Brazil while on an official trip in December 1986.

"I have been suffering a 40 percent disability in my leg muscles and will be physically unable to handle more than 200 problems daily in my congressional office and at the same time carry on a yearlong bruising campaign in the southern Illinois district," Gray said.

His press secretary, Robert Otterson, said the muscle ailment is not life-threatening but has left Gray fatigued.

"It has caused him to slow down quite a bit," Otterson said. "There is no limping or anything like that."

Gray was first elected to the House in 1954 and served for 20 years, until retiring for health reasons. In 1975, he was succeeded by fellow Democrat Paul Simon, now a senator and a contender for the Democratic Party's presidential nomination.

When Simon was elected to the Senate in 1984, Gray came out of retirement to win back his old seat.