Donald I. Baker apparently will leave his post as the nation's top antitrust official at the end of May, if not before.

Notice of his imminent departure came in a statement issued over the weekend by Attorney General Griffin Bell following a meeting of the two men. Baker had asked to see Bell after Rep. John Breckinridge (D-Ky.) told reporters he had been informed by President Carter that Baker would be leaving soon.

In his statement, Bell insisted that Baker "isn't being fired.

"I am well satisfied with his work as head of the ANtitrust Division. I have asked him to stay until May 31," Bell said.

Baker, who declined to talk to reports about the situation, is said to be undecided whether to stay until then.

A holdover from the Ford administration, Baker had been asked last month by Bell to stay on at at least until the summer. Though no commitments were made on either side, the possibility that Baker might be asked to stay on was left open as Bell is said to have told him not to make any commitments to Cornell University, from which he has a leave of absence, before they talked again in April or May.

One atitrust lawyer who had been offered Baker's job in early February, Gordon Spivack, of New YOrk, had turned it down.

Though Breckinridge's comments may have accelerated the process, division sources say Bell's disclosure that Richmond attorney John Shenefield would be named one of Baker's deputies indicated htat "things weren't looking all that optimistic" for Baker's continued tenure.

Shenefield, a member of the Richmond firm of Huntor and Williams, played a major role in at least two recent major antitrust cases, taking a position diametrically opposed to the Antitrust Division's. In a case in which the Supreme Court decided that regulated electric utilities are not exempt from the antitrust laws, Shenefield had argued for his clients, General Public Utilities Corp. and others, that the antitrust laws should not apply to regulated industries.

In another important antitrust case, Shenefield represented the Fairfax County Bar Association in a suit brought by Mr. And Mrs. Lewis A. Goldfarb charging that the minimum fee schedule issued by the Fairfax Bar and enforced by the Virginia State Bar violated the antitrust laws. The Supreme Court ruled in 1975 that the professions did not have the automatic exemption from the antitrust laws they believed they had, and that the fee schedule violated the law.

Sources say Baker balked when told that Shenefield, who aidde the Carter campaign in Richmond and is said to be a White House choice for the job, was to take Jonathan Rose's position as the deputy assistant attorney general philosophies and record on antitrust issues made that unacceptable. It then was determined that Shenefield would take Joe Sims' position as deputy assistant attorney general relate, policy planning and evaluation matters, and Sims would move into Rose's spot.

"I like him and think he's a reasonably able lawyer," a Washington attorney said of Shenefield yesterday, "but his philosophy makes him absolutely the wrong person for a high position in the Justice Department's Antitrust Division."

If Shenefield is being groomed to succeed Baker - as some believe - he would not have "clear sailing for confirmation," a Hill source adds.