Washington (CNN) -- Former Secretary of State Alexander Haig has been admitted to a hospital in Baltimore, Maryland, a hospital spokesman said.
Haig, 85, is at Johns Hopkins Hospital, said the spokesman, Gary Stephenson.
He was admitted on January 28 and is in critical condition, Stephenson said.
Haig was a top official in the administrations of three presidents -- Richard Nixon, Gerald Ford, and Ronald Reagan.
He served as Nixon's deputy assistant for national security affairs beginning in 1970, then in January 1973 became vice chief of staff of the Army.
"His departure from the Nixon administration proved to be short lived," the Nixon Presidential Library and Museum Web site says in a biographical note on Haig. "Four months later, on May 4, 1973, he returned to the White House as chief of staff at the request of the president to fill the vacuum created by H.R. Haldeman's resignation on April 30."
Nixon's resignation came that August.
Haig then became supreme allied commander over NATO forces in Europe until 1979.
He left the military for the private sector, but returned to serve as Reagan's secretary of state in 1981. He resigned the following year.
Haig ran an unsuccessful bid for the 1988 Republican presidential nomination.
Alexander Meigs Haig Jr. was born December 2, 1924, in Bala Cynwyd, Pennsylvania, a suburb of Philadelphia. He attended the University of Notre Dame for two years before transferring to the U.S. Military Academy in 1944. After his graduation in 1947, he served in Japan. He later served under Gen. Douglas MacArthur's staff in Japan during the Korean War.