A preliminary inquiry by the Federal Bureau of Investigation into the activities of former White House deputy chief of staff Michael K. Deaver will include Deaver's role in obtaining a letter from President Reagan endorsing the return of major league baseball to Washington, government sources said yesterday.

Deaver has said he volunteered the free services of his firm, Michael K. Deaver & Associates, to help the D.C. Baseball Commission make a presentation to the major league baseball owners late last year about bringing a team back to Washington. As part of the presentation, the Deaver firm prepared a pamphlet that included a letter from the president endorsing the idea of locating a team here.

The preliminary FBI investigation is the first step in the Justice Department effort to determine whether to appoint an independent counsel to probe allegations that Deaver violated federal conflict-of-interest laws. Deaver himself and five Democratic senators have asked for the appointment of the independent counsel.

In the letter seeking such a probe, the five senators cited questions about Deaver's lobbying activities on the acid-rain issue, on the B1 bomber, on a Puerto Rico tax issue and on behalf of South Korean interests. The baseball issue was not among those mentioned by the senators.

However, White House counsel Peter J. Wallison told presidential aides in a memo this week that the FBI was also looking into the circumstances surrounding the baseball letter. Wallison urged all Reagan's aides to cooperate with the FBI investigation. Other officials said they have been asked to send material on any of the five topics to Wallison and that the FBI may later conduct interviews.

It could not be learned why the FBI has added the baseball letter to the subjects being investigated. New York Times columnist William Safire recently questioned whether Deaver was seeking to generate business from the baseball franchise later by providing free services on the issue.

How the ethics laws apply to the letter situation is also unclear. The law includes restrictions on the contacts that an ex-official such as Deaver may have with his former office, although these limits apply to contacts in behalf of a "particular government matter" in which the office has a "direct or substantial interest."

A source familiar with the activities of the Deaver firm said former White House counsel Fred F. Fielding had given prior approval to sending the baseball letter.

Deaver said in an interview earlier this year that he donated about $20,000 worth of in-kind services by his firm to the baseball effort. He could not be reached for comment yesterday.

The source familiar with the firm's activities said the baseball commission approached Deaver last October seeking help with a presentation to major league owners on why Washington would be a favorable location.

"Deaver said it was not a project the firm could take on" but volunteered the firm's services, the source said. Deaver's employes then created a "marketing booklet" on Washington as a potential location for a baseball franchise, the source said.

The booklet included Reagan's letter, addressed to the owners, the source said. Deaver's firm never had a contract with the commission and never received payment for expenses, the source added.

The source also said, "Any suggestion that the work was done with regard to future work is totally without foundation." Deaver did not personally obtain the letter from Reagan, but someone in his firm did, the source said.

The letter was a revised version of one Reagan sent in September 1984. The booklets and a script for the presentation were then turned over to D.C. Baseball Commission members, who actually made the pitch in New York, the source said.