National Arts Endowment chairman Livingston L. Biddle has informed six of the agency's 13 program administrators that he intends to replace them under a new plan to limit tenure in such positions to five years.

The program grant directors are key members of the Endowment bureaucracy, supervising the spending of about $96 million in 1978, or four-fifths of the total budget. Two have held their posts since the agency was created 12 years ago.

Biddle confirmed yesterday that he informed the administrators of his decision at a meeting with them three weeks ago. He said that two other program directors, whose assignments had changed during more than five years of service, might also be replaced. The remaining five administrators would remain in their posts.

"I think this is necessary because of the dangers of over-bureaucratization in the arts," Biddle said yesterday. I'm not saying that any of the program directors are not doing a good job. And I realize that when many of them took their jobs they had no reason to believe these would not be career jobs.

"But I believe in the principle of keeping fresh life coming into the Endowment. This will insure regular feeding of fresh ideas from the field. And it will also give protection against any program becoming too much the product of a single individual's point of view."

He said the program directors have "immense individual responsibility."

Leonard Randolph, director of the literature program since 1970, said yesterday that he had been planning to resign for some time anyway.

"I appreciate the necessity in any job in having fresh ideas, a new face and new personalities," he said. "I think the unfortunate thing is that there are people who have been here for a number of years who feel there is a large amount of ingratitude for what they have done. But I don't think Liv feels that way."

Walter Anderson, the veteran music program grant administrator, declined to analyze the Biddle plan yesterday, but added "I don't know whether any ideas are a shock to me any more. We live in a world where things come spontaneously."

Biddle says no one will be replaced until they "have a proper place to go." Conceivably, other jobs could be found for some in the Endowment. No deadlines are to be set, "but I would hope that we would be making progress on this by the end of summer at least," said Biddle.

Biddle said members of the group appealed to him Wednesday to come up with an alternative proposal, and he has given them a week in which to do so.

The directors who would immediately be affected are John Kerr, education; Vantile Whitfield, expansion arts; Randolph, literature; John Spencer, museums. Anderson, music, and Ruth Mayleas, theater.

Brian O'Doherty, media arts; and Gordon Braithwaite, special projects, served as program directors for more than five years, though not in their current assignments. The five other directors are Roy Knight, architecture; Rhoda Graver, dance; Henry Putsh, federal-state partnership; Jim Melchert, visual arts, and Bess Hames, folk arts.

About 5,500 program grants were handed out last year. Four times that many applications were screened by the program directors, and the expert advisory panels that they employ to make the final decisions.

Biddle argues that this change will put the program directorships on the same principle of "rotation" between the private sector and the government that already applies in other areas of the Endowment.

Members of the 26-member advisory council, for example, are limited to six-year terms. Biddle yesterday repeated an earlier statement that, though he would accept a second four-year term in his own past, "it is simply not proper for a chairman to serve for more than eight years."