Just before Celeste Holm walked into the mental ward of a World War II hospital to perform for shellshocked soldiers, the nurse said she hoped the singer didn't mind being locked in. "She said no one knew what these men had gone through," Holm recalled at a reception for the World Congress for Mental Health last night.

"So I sang. The men just sat on their beds. I came near the end, and then this guy, he smiled at me. I was so startled! When I got out, the nurse threw her arms around me and said, 'That's the first response we've gotten from him in six months.' "

One friend's claim that "Holm began with mental health before Freud thought of it" may be a little bit of an exaggeration. But since her first visit to that mental ward some 40 years ago, when she was singing in the original "Oklahoma!," Holm has traveled around the country as a volunteer, speaking on behalf of mental health services.

At the Blair House reception, "volunteer" was the magic word, especially for Rosalynn Carter and Health and Human Services Secretary Margaret Heckler. The former first lady interrupted the initial editing of her book, scheduled to appear next spring or summer, to speak at the final session of the annual congress. Heckler took time off from what one assistant called "AIDS month" to sponsor the reception. And 185 World Congress participants stood in line for 40 minutes to shake the two women's hands.

While Heckler and Carter were working the receiving line to the sound of a harp and the click of bearded psychiatrists' cameras, a hot and tired Lady Juliet Bingley, board chairman of the British volunteer mental health organization MIND, sat out on the patio.

"Mrs. Thatcher doesn't understand the volunteer movement at all," she said. "She thinks it should work as a supplement to what in fact is a statutory responsibility. We can provide alternatives, not replacements. And she's been voted in again, the silly fools. She's putting an awful amount of money into defense, you know. She and Reagan are good old buddies," Bingley said with a loud laugh and a shake of the head.

"AIDS is still our number one priority," Heckler said as the last guest left the reception line. "Until it's resolved, this is a disease that has to remain on the front burner. But of course, there are so many problems I'm concerned with . . . Drunk driving. Mental health. And volunteers can do more than we think on these issues."

Carter first got involved in mental health causes when voters in her husband's 1971 campaign for governor of Georgia raised the issue. "I would go home and ask Jimmy what he was going to do about it when he became governor," she said. "I worked on the issue for four years here. I worked on a good program and then it was just gone."

She smiled the familiar smile and gave a faint shrug of the shoulders. "Our legislation didn't call for that much money. Well, our study is still there, and I hope someday it will be implemented."