Former attorneys general John N. Mitchell and Richard G. Kleindienst both testified in federal court here yesterday that the FBI did not ask them for approval to conduct secret entries at private homes -- called black bag jobs -- during their tenure at the Justice Department in the late 1960s and early 1970s.

Mitchell and Kleindienst, both prosecution witnesses at the conspiracy trial of two former high-ranking FBI aides, also testified, however, that they would have authorized such surreptitious entries -- without court warrants -- in appropriate national security cases.

The two FBI officials, W. Mark Felt, the former No. 2 man at the bureau, and Edward S. Miller, once the head of the Domestic Intelligence Division, are charged with illegally authorizing break-ins directed at the homes of friends and relatives of fugitive members of the radical Weather Underground organization in 1972 and 1973. Kleindienst was attorney general at that time.

During yesterday's proceedings in U.S. District Court, the government also called former attorney general Ramsey Clark, who testified that in 1968 he turned down a request from the bureau for approval of a surreptitious entry involving a national security case identified at the trial only as "Program C." Clark, who was attorney general from September, 1966, to January, 1969, also testified that he was never specifically asked by the bureau to approve a black bag job purely to search for information.

The prosecution, which called all three men as rebuttal witnesses yesterday, appeared to try to use their testimony to reinforce the government's earlier evidence that the bureau concealed information about black bag jobs from the Justice Department and never sought the approval of the attorney general before the entries were carried out. The defense completed presentation of its evidence Monday. The trial is now in its seventh week before Chief Judge William B. Bryant.

The prosecution contends that warrantless entries approved by Felt and Miller were illegal because they were directed at innocent persons with no significant connections to any foreign powers, connections that might otherwise have provided sanction for the break-ins as national security cases. Even it the entries involved national security matters, the government maintains, the FBI needed approval on a case-by-case basis from the president or the attorney general. Both Mitchell and Kleindienst testified that they knew of no law that required the bureau to get their approval before conducting such entries.

Kleindienst testified that if he had known that the FBI was conducting such entries, he would have required the bureau to first obtain his authorization on a case-by-case basis.

But Mitchell, who was Kleindienst's predecessor and attorney general from January 1969 to March 1972, testified it was his understanding that such entries in the past had been carried out under the FBI's own authority, and that requests were not even submitted to the attorney general for approval.

Mitchell's appearance at U.S. District Court yesterday was the first time he has testified from the witness stand in a court case since he was convicted in the same courthouse in early 1975 of criminal charges in connection with the Watergate break-in.

Kleindienst and Mitchell both testified that, based on their knowledge of Weatherman activities related to hostile foreign powers, they probably would have approved warrantless searches for national security reasons. Clark said he would not approve such searches under any circumstances.

Kleindienst testified that even in a case in which the targets of such searches were friends and relatives of a radical organization, such as the Weathermen, he would have authorized warrantless entries "without hesitation" in order to gather intelligence information if he was satisfied that the case involved national security interests.