Washington, D.C. ? Sen. Robert C. Byrd, who holds the record as the longest-serving member of Congress, is seriously ill in a Washington-area hospital, his office said Sunday.

The 92-year-old West Virginia Democrat has been in the hospital since late last week, his office said in a statement. At first Byrd was believed to be suffering from heat exhaustion and severe dehydration, the statement said, but other medical conditions have developed. His condition was described as serious.

The statement did not name the hospital.