A federal court jury deliberated for five hours yesterday at the conflict-of-interest trial of former White House aide Lyn Nofziger and his partner, Mark Bragg.

U.S. District Court Judge Thomas A. Flannery sent the case to the jurors yesterday after instructing them for 45 minutes about how to deal with the relatively new law underlying the indictment. He sent them home at 5:30 p.m.

Nofziger, a friend and confidant of President Reagan for more than 20 years, was charged with illegal lobbying at the White House on four occasions in 1982 -- twice for the now-bankrupt Wedtech Corp. and twice for other clients. Bragg, Nofziger's partner in a Washington consulting firm that they established that January, was indicted on one count of aiding and abetting Nofziger on Wedtech's behalf.

The 1978 Ethics in Government Act under which they were charged prohibits former senior government officials from lobbying at their old agencies for a year after leaving on any "particular matter" that is of "direct and substantial interest" to the agency.

Flannery defined those untested phrases in a way that made it appear independent counsel James C. McKay had prevailed on some key points in the backstage debate that began Monday over the jury instructions. The trial centered on these issues far more than it did on matters of fact, although defense lawyers gave no indication that they were unhappy when the judge defined the phrases.

Flannery pointed out that the "particular matter" alleged in the Wedtech counts was a multimillion-dollar Army engine contract that the Bronx-based company was seeking when it hired Nofziger and Bragg early in 1982. In the case of another client, the Marine Engineers Beneficial Association (AFL-CIO), the judge pointed out that expanding the use of civilian crews on noncombatant U.S. ships was the alleged "particular matter" and in the case of Fairchild Republic Corp., it was funding for continued production of Fairchild's A10 aircraft.

Nofziger's lawyers had suggested all these issues were too small and niggling to have been covered by the ethics law, but Flannery told the jurors that "a 'particular matter' is not a matter of general concern like poverty, unemployment or housing," but instead is a specific project.

The judge added that White House interest would be "direct" if "an officer, employee or other representative of the White House was actively concerned with the matter," even if final action was up to some other federal agency.

To determine "substantial" interest, the judge said that the jurors could consider the effort devoted to an issue at the White House level and the importance attached to it there. But he also told them that "it is not necessary for you to find that a particular matter was of major importance to the White House as compared to other matters."

One of the original members of the panel, a retired schoolteacher, was excused after Bragg's lawyer, Richard Ben-Veniste, complained at a bench conference that she had fallen asleep during his summation Tuesday. She was replaced by an alternate.

Nofziger appeared for final arguments Tuesday morning holding a velveteen Mickey Mouse doll someone had sent him and making observations about Vice President Bush's poor showing in the Iowa caucuses. Yesterday he seemed unusually somber as he entered the courthouse.