Eugene Ormandy, after 43 years as music director of the Philadelphia Orchestra, is preparing to relinquish that title at the end of the 1979-80 season.

In a letter read yesterday to the orchestra musicians, Ormandy, 79, said that at the request of the orchestra association board, he would assume the title of laureate conductor at that time.

He said he will participate in a number of weeks of subsequent Philadelphia Orchestra seasons and will appear with other orchestras in the United States and abroad.

"After 1979-80," he said, "I will have spent 44 years as music director of the orchestra. My association with the artists and musicians of the orchestra has been my life. I could not envision any but a gradual lessening of this close affiliation and I am delighted to be able to continue to contribute to this great orchestra in any way possible."

Ormandy will celebrate his 80th birthday Nov. 18 during the orchestra's 80th season. In his years with the Philadelphia, he has engaged every one of the players now in the orchestra. His tenure has been by far the longest of any conductor of an American orchestra.

Scheduled to conduct 13 weeks in the coming season, Ormandy will end his directorship with one of the gala concerts the Philadelphians traditionally give to close their summer seasons at Saratoga, N.Y.

While details concerning Ormandy's successor to one of the world's choicest conducting positions have yet to be settled, there is no question that it will be Riccardo Muti, the young Italian conductor who has held the title of principal guest conductor with the orchestra in recent years, the first person ever to be given that label. He was hand-picked for the spot by Ormandy.

At yesterday's meeting it was simply said that "final details concerning arrangements with Mr. Muti would be announced shortly."

Ormandy's years in Philadelphia saw him raise the orchestra to the point where it was often described, both in this country and Europe, as "the world's greatest," or, in a phrase coined by Paul Henry Lang of the New York Herald Tribune, "the solid gold Cadillac of orchestra." Its concerts at home and on tours both domestic and abroad, and its recordings, made the Philadelphia Orchestra one of the world's busiest and most prosperous.