Sen. Lee Metcalf of Montana, a liberal Democrat whose causes included consumer interests, conservation and education, was found dead yesterday in his apartment in Helena, Mont.He was 66.

The senator's Washington office announced that he had died during his sleep sometime Wednesday night and that his body was found by his foster son, Jerry. No cause of death was given, but Sen. Metcalf had been in uncertain health in recent years and had a heart condition.

His death came 25 years after he first entered Congress as a member of the House of Representatives, and a year before he planned to retire after completing 18 years of service in the Senate.

In his Washington years, Sen. Metcalf gained a reputation among his colleagues as one of the last of the traditional western populists. He favored consumers over coal companies and power companies; wildlife and wilderness over economic interests. He was one of the major forces behind efforts in recent years to give Congress the means to play a larger and more coherent role in the setting of national policy.

From his days in the House, in which he served from 1953 until 1961, he favored federal support for education, the the expansion of federal assistance for the elderly and the disabled, and similar measures.

At the time of his death, he was a member of the Senate conference committee considering the removal of price controls on natural gas as part of President Carter's energy program. The Senate confereees have been deadlocked 9 to 9. Sen. Metcalf favored continuing controls. It was not clear last night how or whether his death would effect that issue since his replacement on the committee had not been named.

President Carter said in his news conference yesterday that the impasse among the Senate conferees was stalling his energy program. At the time, the President did not know of Sen. Metcalf's death.

Sen. Metcalf brought to his work what his colleagues regarded as one of the best and most disciplined intelects in Congress. A lawyer by training, a former assistant attorney general of Montana and a former associate Justice of his state's Supreme Court, he was noted for his ability as an extemporaneous speaker. He also had a reputation for fair-mindedness.

In recent years, the senator had become known as a heavy drinker. He said recently that while he was "noteetotaler," he took "a lot of medication" because of a knee injury suffered while playing football in college, and which had been aggrevated by World War II service in the Army. He had carried a cane in recent years.

Apart from physical infirmities, Sen. Metcalf found the life of a senator in some ways less rewarding than that or a representative, according to associates. For one thing, he spent most of his Senate years under the shadow of his fellow Montanan, Mike Mansfield, the former Democratic majority leader. For another, Sen. Metcalf believed that the procedures of the Senate were less conducive to accomplishment than those of the House.

He began his House career with two notable victories. One was the defeat of a grazing bill that, in his opinion, would have allowed private interests to take over public lands. The other was the defeat of a bill that he believed would have given unwarranted privileges to timber companies.

In 1958, he was one of the authors of the National Defense Education Act. This was part of the nation's response to the launching by the Soviet Union of Sputnik, the first satellite to be put into orbit. The act was the first to provide substantial federal funds to improve public education with particular emphasis on science, technology and languages.

Over the years Sen Metcalf supported other educational programs, including extensions of the GI Bill to Vietnam veterans. He supported proposals to ensure clean air and water, fair packaging and labeling laws for consumers, and measures to make government more responsive.

In 1973, he conducted hearings that led to the passage a year later of the Congressional Budget and Impoundment Control Act. This law establishment a congressional budget office and two budget committees.

Its effect has been to give Congress an overall view of government spending, deficits and tax revenue. Previously, Congress had acted on revenue measures on a piecemeal basis.

At news of the senator's death, flags at the White House were lowered to half staff.

Former Sen. Mansfield, now the U.S. embassador to Japan, said in Tokyo that his former colleague had been "a superb senator, the best partner in the Senate I ever had."

Sen. Robert C. Byrd (D-W.Va.), Mansfield's successor as Senate majority leader, said Sen. Metcalf had been "an outstanding senator and a good man."

Sen. John Melcher (D), Montana's other senator, said that the death was "a great loss for the people of Montana and the people of the country that have long appreciated the value of his service and leadership."

Sen. Frank Church (D-Idaho) called Sen. Metcalf "a very dear friend and a great western senator."

Sen. Metcalf's interim replacement will be named by Gov. Thomas Judge of Montana, a Democrat.

Sen. Metcalf was born in Stevensville Mont. He attended Montana State University for a year and then transferred to Stanford University.He earned a bachelor's degree in history and economics from Standard in 1936, and in the same year, earned a law degree from Montana. He was elected to the Mountain House of Representatives in November, 1936.

In the following year, he became an assistant attorney general of Montana. He resigned in 1941, and practiced law briefly in Hamilton, Mont. In 1942, he enlisted in the Army, saw service in Europe and was discharged in 1946 with the rank of first lieutenant. Later that year, he was elected as associate justice of the Montana Supreme Court. At the expiration of his six-year term, he won his first election to Congress.

Apart from his legislative activities in the House, Sen. Metcalf founded and was first chairman of the liberal Democratic Study Group. Since 1963, he had held the largely honorary post of acting president pro tem of the Senate.

In addition to his foster son, Sen. Metcalf's survivors include his wife, the former Donna Hoover, whom he married in 1938, and his mother, Rhoda Metcalf.