Rep. Adam Benjamin Jr., a third-term Indiana Democrat and chairman of the congressional steel caucus, was found dead in his one-room Capitol Hill apartment yesterday. Officials said Benjamin, 47, died of natural causes.

D.C. Deputy Medical Examiner Douglas S. Dixon said an autopsy revealed that Benjamin had high blood pressure and apparently died of a heart attack.

There was no indication of foul play, Dixon said. Benjamin apparently died some time over the weekend, but the exact time of death has not been determined.

The congressman's body was discovered yesterday morning by an aide, Peter Visclosky, who went to Benjamin's apartment in the 100 block of C Street SE after Benjamin failed to appear at his 13-year-old son's opening school ceremony at Georgetown Preparatory School on Sunday and failed to arrive at Bethesda Naval Hospital for an eye examination scheduled for 9 a.m. yesterday.

Sandra Kormos, a spokesman in the congressman's office, said Benjamin and his son, Adam III, arrived in Washington from Indiana on Saturday. That night, Benjamin phoned his wife, Patricia, at their home in Hobart, Ind., and met with Visclosky at the congressman's office on Capitol Hill, Kormos said.

Janet Schuessler, a former aide and friend of Benjamin's, said the congressman told her he had high blood pressure and was overweight.

East Chicago, Ind., Mayor Robert Pastrick, a Democrat, said Benjamin "worked himself to death." Republican Lt. Gov. John Mutz said, "No one worked as many hours for his constituents."

Sen. Richard Lugar (R-Ind.) said, "Adam Benjamin was a distinguished Hoosier legislator whose mastery of detail and faithfulness to duty produced solid achievement. . . .I will miss him deeply as a strong legislative ally and as a gracious friend."

Former Indiana senator Birch Bayh said, "To me, Adam Benjamin was more than a congressman. He was a dear personal friend and a dedicated, self-sacrificing public servant. When John Kennedy said 'Here on earth, God's work must truly be our own,' he must have had Adam Benjamin in mind, for Adam had been doing God's work for all of us for most of his life."

Gary, Ind., Mayor Richard Hatcher said, "He was so popular that he rarely had any significant opposition. That's the greatest tribute that can be paid to any public official, that no one feels inclined to run against him."

Indiana's Republican Gov. Robert Orr ordered flags on all state buildings lowered to half-staff, and said, "friends who have served with him in government universally appreciated his special talents."

Benjamin was a strong political force in Indiana's 1st Congressional District, the heavily industrial northwest area of the state. Benjamin was unopposed in the Democratic primary this May and was heavily favored to defeat Republican opponent Thomas Krieger in a bid for a fourth term this November. The Indiana Democratic Central Committee is now researching a way to fill the vacancy on the ballot.

Benjamin earned an engineering degree from the U.S. Military Academy at West Point and a law degree from Valparaiso University in Indiana. He also was to have been awarded a master's degree in comparative and international law from Georgetown University, where he had been lecturing on the social sciences and budget studies for the past two years.

Benjamin served with the Marine Corps in Korea and later with the Army.

After four years in the Indiana House of Representatives, Benjamin won his first term in Congress in 1976 by defeating House Rules Committee chairman Ray J. Madden.

In Congress, Benjamin served on the House Appropriations and Budget committees and was chairman of both the executive committee of the congressional steel caucus and the House Transportation Appropriations subcommittee. He was a member of the Democratic Steering and Policy Committee and the Democratic caucus' Committee on Party Effectiveness.

Benjamin's body will be flown back to Indiana, where the funeral will take place. He is survived by his wife, the former Patricia Ann Sullivan, and three children, Adam III, Alison and Arianne.