The acting chief of the Drug Enforcement Administration acknowledged at a grueling Senate confirmation hearing yesterday that he consciously withheld information about the FBI investigation of Secretary of Labor Raymond J. Donovan in 1981, but said he was not trying to mislead anyone.

Francis M. Mullen Jr., who was in charge of the Donovan investigation before his appointment to DEA, told the Senate Judiciary Committee that if he had it to do all over again, "I would have asked for an executive session" to provide the details that were held back.

Testifying under oath, Mullen declared repeatedly, however, that "I did not deliberately withhold anything in order to mislead the committee" that had been considering Donovan's nomination.

"In reviewing the transcripts of my testimony," Mullen said, "I can see how it would appear to some that may be the case . . . . There may have been some misunderstanding but I can only personally assure you that I would never intentionally mislead the committee. There was just no good reason to do that and no one with any sense at all would do that."

Sen. Howard M. Metzenbaum (D-Ohio) said he was "astounded" by Mullen's 1981 performance and "shocked" that the Reagan administration would send him back up to Capitol Hill as its choice as DEA administrator. However, the long-delayed appointment still appeared to be assured.

The clincher came from Sen. Orrin G. Hatch (R-Utah) who announced after persistent questioning of Mullen that he was still "deeply offended" by the longtime FBI official's performance but that he was going to give him "the benefit of the doubt."

"But if you're ever asked before a congressional committee again," Hatch warned, "you do your homework and don't you withhold information consciously or otherwise, or by mistake or otherwise."

It was the Senate Labor Committee where Hatch is chairman that handled the Donovan nomination. Mullen's nomination as DEA chief has been held up at the request of Hatch and Sen. Edward M. Kennedy (D-Mass.) since shortly after the publication last year of an article in The Washington Post recounting a tape-recorded interview with Mullen.

In it, Mullen acknowledged that the FBI knew Donovan had been mentioned on an organized crime wiretap at the time of Donovan's confirmation hearings, but withheld the information from the Labor Committee because, Mullen said then, the conversation was "not pertinent" to the FBI's background investigation.

Mullen said yesterday that this was a bad choice of words. He told the senators that the reason he didn't mention the wiretap, involving a conversation between a reputed Mafia soldier and his subsequently murdered son, was because Mullen considered it repetitive of other allegations that were reported to the Labor Committee concerning alleged ties between Donovan and organized crime figures.

Hatch took exception to that, pointing out that the FBI did not report similar allegations to the committee until late January, 1981, more than 10 days after FBI headquarters learned about the wiretap mention.

A Judiciary Committee spokesman said the vote on Mullen's nomination may occur today.

For his part, Secretary Donovan, a former New Jersey construction company executive, steadfastly denied any ties between himself and organized crime figures. A special prosecutor concluded last year that there was "insufficient credible evidence" to support prosecution on any score.