With former President Reagan ruled out as a witness, Oliver North's lawyers must look for other ways to show that his superiors authorized his actions in the Iran-Contra affair.

The defense begins presenting its case Monday at North's trial on charges ranging from lying to Congress to receiving an illegal gratuity.While prosecutors were presenting their side over the last six weeks, North's lawyers brought out a number of secret actions taken by Reagan while in the White House on behalf of the Contra rebels fighting to overthrow Nicaragua's Sandinista government.

But that failed to persuade U.S. District Judge Gerhard A. Gesell that Reagan should be called to the stand as requested by North's lawyers.

On Friday, Gesell declared that "the trial rec-ord presently contains no proof that defedant North ever received any authorization from President Reaan to engage in the illegal conduct alleged, either directly or indirectly, orally or in writing."

That represented a blow to North's strategy but is not expected to deter his lawyers from focusing on Reagan.

They still may draw on secrets contained in a 44-page admission by the government, outlining the administration's inducements to other countries in exchange for help to the Contras.

In addition, they are expected to zero in on exhibits already presented at the trial suggesting authorization.

For example, after North denied in an Aug. 6, 1986, meeting with the House Intelligence Committee that he was aiding the Contras, then-national security adviser John Poindexter wrote his subordinate: "Well done." North now is accused of obstructing Congress in connection with the meeting.

The defense subpoenaed Poindexter, a co-defendant in the Iran-Contra case who will be tried separately from North later this year. Asserting his constitutional rights, Poindexter is declining to appear.

In his absence, North's lawyers want to show several hours of televised testimony Poindexter gave on Capitol Hill in 1987 during the congressional Iran-Contra hearings.

Poindexter testified that his one regret was that he didn't follow up personally after North reported a threat on his life by terrorist Abu Nidal. Part of North's defense to the charge of accepting an illegal gratuity, a home security system, is that he feared for his family's safety and that the government wouldn't pay for a security system.

The office of independent counsel Lawrence Walsh opposes introducing the Poindexter videotape.

The problem with focusing on Reagan is that North's lawyers have been unable to place him at the scene of any of the alleged crimes. The closest brush to date involving the president came in the testimony of former national security adviser Robert McFarlane.

McFarlane said he had talked with Reagan in the Oval Office about congressional inquiries before sending letters to Congress in September 1985 which denied North was assisting the Con-tras. McFarlane later pleaded guilty to misdemeanors in connection with the letters, which North is accused of preparing and for which he is now charged with felonies.