The son of Navajo tribal Chairman Peter MacDonald told a Senate panel Monday he helped devise a plan to cover up questionable payments his father received from a friend who brokered the $33 million sale of a ranch to the tribe.

Peter MacDonald Jr., a lawyer from Phoenix, testified under immunity from prosecution at the beginning of a second week of hearings by the special Senate committee, which is probing a wide range of reported corruption on Indian reservations.The elder MacDonald also was subpoenaed to testify, but informed the committee he would invoke his Fifth Amendment privilege against self-incrimination if compelled to appear because he was not granted immunity.

Sen. Dennis DeConcini, chairman of the panel, said the committee denied immunity to the Navajo chairman because of objections from the Justice Department. He said the department had raised no such objections about any other committee witness.

The activities of the elder MacDonald, the elected head of America's largest Indian tribe, are one subject of the committee's inquiry, as well as the focus of a federal grand jury investigation in Phoenix.

Monday's hearing, like the grand jury probe, dealt specifically with the purchase of the Big Boquillas Ranch by the Navajo Nation for $33.4 million in July 1987. The same ranch was sold for $26.2 million a day earlier to Tracy Oil & Gas Co., run by Phoenix oil man Tom Tracy.

The back-to-back sales were brokered by real estate agent Byron "Bud" Brown, a friend and major political fund-raiser for the elder MacDonald, according to testimony from Eugene Twardowicz, an investigator for the Senate committee.

MacDonald's son testified that after federal investigators began looking into the deal, he, his father and Brown held a series of meetings in 1988 to concoct the cover-up of a $25,000 payment and a luxury car lease the Navajo chairman accepted from Brown.

The elder MacDonald is in his 14th year as chairman of the 200,000-member tribe.