Secretly, in the spring of 1976, a soft-spoken millionaire businessman, known to his associates as a "workaholic," gave a $33,000 loan to a financially pressed friend, who happened to be one of the most powerful officials in the District of Columbia's fledgling home-rule government.

At about the same time, the two men and their aides negotiated a controversial lease under which the D.C. government agreed to pay $5.6 million over a 20-year period to rent a drab two-story Northeast Washington office building from a partnership controlled by the wealthy business executive.

Were these "parallel actions," as federal prosecutors contend, demonstrating the existence of a corrupt conspiracy by the two men - businessman Dominic F. Antonelli Jr. and former city official Joseph P. Yeldell - to defraud the District of Columbia for their mutual benefit?

Or were these "wholly separate" and "totally unrelated" events, as defense lawyers assert, proving nothing more than the existence of a longstanding friendship between two honest and civic-minded men who have never engaged in wrongdoing?

In the past two weeks, the prosecutors have, as is common in conspiracy trials, assembled an enormously detailed but largely circumstantial case in their efforts to convince an all-black jury that the profit-minded real estate developer bribed and conspired with the debt-ridden city official "to defraud the District of Columbia of an concerning its lawful governmental functions," as a grand jury indictment alleges.

Now the defense has started to present its case to the U.S. District Court jury in an attempt to disassemble the picture that the prosecution has drawn. Antonelli was called by his chief trial lawyer, Edward Bennett Williams, as the first defense witness on Friday, and the defense presentation is expected to continue for much or all of this week. Yeldell also is expected to testify.

Throughout the courtroom drama, Antonelli and Yeldell have appeared relaxed. Antonelli, 56, a grayhaired, conservatively dressed businessman who wears darkly tinted glasses because of eye ailments, has chatted amiably with Yeldell, 46, a hand-slapping politician who wears dapper three-piece suits and often is surrounded by friends in a court house corridor during recesses in the trial. Yeldell is now on unpaid leave from his most recent job as a top aide to Mayor Walter E. Washington.

Time appears to constitute a key element in the prosecution's charges. The prosecutors have sought to show, in part, that Antonelli's efforts to obtain a lease from the D.C. Department of Human Resources, the agency Yeldell headed at the time, coincided with a period in which Antonelli first guaranteed a series of short-term bank loans for Yeldell and then gave Yeldell the secret $33,000 loan.

The prosecutors have pointed to what the portray as an incriminating succession of events:

In the summer of 1975, Yeldell first showed interest in renting a building at 60 Florida Ave. NE for his huge health and social service agency, DHR, at about the same time that Antonelli first expressed interest in buying the property from its previous owner, Peoples Drug Stores.

In October 1975, Yeldell temporarily halted negotiations for the Florida Avenue building after learning that the D. C. Department of General Services, which was handling lease arrangements for DHR, has decided to reject Antonelli's lease proposals and to negotiate a lease for the same property from another businessman, Emanuel Logan.

Immediately after DHR was granted independent powers to rent privately owned buildings in December 1975, Yeldell called Antonelli to his office to start lease negotiations once again over the Florida Avenue building.

During the first few months of 1976, Yeldell turned over allegedly confidential city information about the proposed Florida Avenue lease to Antonelli, hired a negotiating consultant recommended by Antonelli and cleared aside other obstacles to conclude the lease with Antonelli' partnership at about the same time that Antonelli provided the $33,000 loan for Yeldell. The loan was kept secret, because it was recorded in the name of a "straw," or fictitious person.

By and large, the defense appears to have acknowledged that the events described by the prosecution did, in fact, take place on approximately the same days the prosecution contends they occurred.

But defense lawyers and the first defese witness, Antonelli, have asserted that Antonelli's financial assistance to Yeldell was prompted by friendship with no expectation that Yeldell would grant him any favors in return. They have also argued that the Florida Avenue lease was fairly and properly negotiated and that its terms were advantageous to the city.

When Antonelli's and Yeldell's actions appeared to follow parallel courses, the defense has contended, each of the two men frequently had no idea what the other was doing or that their actions might somebody converage.

How this array of intertwining events and conflicting explanations about their meaning has been viewed from the vantage point of the jurors is unknown. They have been sequestered at a local motel since the start of the trial and have been instructed by presiding Judge Gerhard A. Gesell not to discuss the evidence even with one another.