National Medical Enterprises Inc., a leading California-based provider of health-care facilities, has agreed in principle to acquire First Washington Group Inc. of the District for $100 million.

A spokesman for National Medical said the plan to acquire First Washington is subject to a definitive agreement and approval by shareholders of First Washington as well as various regulatory agencies.

Officials at First Washington refused to comment on the agreement with National Medical.

First Washington is a privately held company and the parent of Psychiatric Institutes of America. It operates 1,586 beds at 21 hospitals and psychiatric-care facilities in 13 states.

John Bedrosian, a founder, director and an executive vice president of National Medical, said the acquisition of First Washington would take the Los Angeles-based company "more aggressively into the psychiatric-care field."

"We consider ourselves a total health-care provider and this is just another move to solidify that position," Bedrosian said.

National Medical has completed 33 acquisitions in the past 13 months.

"The health-care field is an area that is consolidating," Bedrosian said. "Acquisitions will be part of the future."

National Medical owns or manages 63 acute-care hospitals with 8,345 beds in several states. It also operates long-term health-care facilities throughout the United States and four hospitals in Saudi Arabia.

For the fiscal year ended May 31, National Medical, which is traded on the New York Stock Exchange, had revenues in excess of $1 billion and net income of $51.8 million, or $1.24 a share.

National Medical estimates that First Washington's revenues will be in the neighborhood of $155 million for the fiscal year ending Sept. 30, 1982.