Ofield Dukes, a Washington-based public relations consultant, testified yesterday that he voluntarily accepted an inflated salary from Rep. Charles C. Diggs Jr. (D-Mich.), in order to use the extra money to pay for newspaper advertisements and radio time purchased by Diggs.

Dukes, called as a prosecution witness in the trial of Diggs on 29 counts of mail fraud and illegally diverting federal money to his personal use, asserted that the raising of his salary to pay the bills was a "legitimate reimbursement process" that he uses for other, nongovernmental clients.

Dukes, who has not been charged with any crime by federal prosecutors, acknowledged that on at least three occasions his normal monthly salary was tripled so that he could pay for Diggs a Detroit newspaper bill, a Detroit radio station bill and a bill at the House of Representatives recording studio.

The government maintains that those payments were made illegally by Diggs. The defense has argued that a later change in the law by the House of Representatives made legal the practice that Diggs employed with Dukes and at least two other congressional employes.

Dukes, a talkative witness who several times was cut off by Assistant United States Attorney Eric Marcy when Dukes attempted to explain the "set of circumstances" that led to a particular payment he had made for Diggs, was instructed by U.S. District Court Judge Oliver Gasch to limit his answers to the question being asked.

On cross-examination by defense lawyer Robert Watkins. Dukes continued to give answers at great length, often going beyond thes cope of the question, "I understand you're trying to be helpful and I appreciate it," Watkins told Dukes at one point. "But there's a form we have to follow to get through this process."

At one point, when Watkins asked Dukes how many times he had met with prosecutors, Duke turned toward prosecutor Marcy and asked, "Mr. Marcy, do you remember?"

When Dukes directed a second question at chief prosecutor John Kotelly, Gasch reprimmanded the witness. "You're not supposed to ask questions. You're supposed to answer them," Gasch told Duke. "If you don't know, say so."

Ultimately, Dukes testified, he decided to end the reimbursement arrangement with Diggs - not because Dukes thought it was improper - but because the arrangement was causing him tax problems. Dukes said he thought nothing was wrong about the arrangement when he ended it, "and I still don't."

Duke was one of three major prosecution witnesses to testify yesterday in the fifth day of the trial as Gasch attempted to conclude it as quickly as possible to accommodate the jury of nine women and three men and five alternates, who have been sequestered.

The other two principal witnesses, Jeralee Richmond and George G. Johnson, both testified that they were paid by Diggs on his congressional payroll, although they did little or no congressional work.

Richmond, a long-time employe of the Diggs family, spent most of her time on the witness time crying as she answered questions. She testified that she returned to work for Diggs' funeral home in Detroit after a seven-year absence and also worked about 20 percent of the time doing consitutent work for Diggs. All of her salary, she testified, came from Digg's congressional payroll.

Johnson, a certified public accountant, testified that Diggs owed him between $2,000 and $10,000 for accounting and tax work done for both Digggs and Diggs' funeral home, the House of Diggs. Following a conversation about the debt with Diggs in 1973, Johnson testified, he began getting payroll checks from Diggs' congressional office. Johnson said that, aside from having conversations with Diggs, about economic development and job opportunities in Detroit's black community, he could recall doing no work for Diggs.

Johnson said he credited the payroll checks against the debt owed by the House of Diggs, although Diggs had not requested him to do so, and Johnson said he now was unable to explain why he did it. "Looking back on it now," Johnson said, "it doesn't sound too intelligent."

Johnson said he ended the arrangement in late 1974 after receiving more than $2,000 in pay because he "couldn't rationalize getting those checks."

The trial resumes Tuesday morning.