First Lady Rosalynn Carter spent an afternoon on the town yesterday.

She walked from her house on Pennsylvania Avenue to the nearest subway stop, tried to stuff a wrinkled dollar bill into an unco-operative Farecard machine, rode Metro's blue line out to D.C. General Hospital, and got her smock dirty painting the emergency room wall.

The First Lady had spoken frequently of her desire to get involved in the human problems of the city of Washington, and yesterday's very public tour marked the first dramatic step in that effort.

The emphasis was on beautification at D.C. General. "People who are poor and have no beauty in their lives should be able to come to a place that is pleasant," said Mrs. Carter. As First Lady, she added, she hopes to spark volunteer programs, and not to become "a conduit for federal funds."

Business as usual, a state of affairs not often or easily interrupted at D.C. General, collapsed completely during the First Lady's 1 1/2-hour visit. Mrs. Carter was engulfed in reporters, nurses, doctors, patients, orderlies and gawkers of all stripes wherever she went.

Her first stop was the emergency room, where she joined a group of volunteers in applying a fresh coat of white paint over the lod layer of drab, institutional green. The paint job was part of an overall hospital redecoration plan utilizing labor and supplies donated by volunteers.

"Private people are going to have to assume responsibility," said Mrs. Carter. "I have found that while Jimmy was governer and since he has been president, if people know a specific need then they will respond to it."

Alice W. Maynard, a hospital volunteer and retired nursing assistant, craned to get a good look at the First Lady, but expressed some skepticism about the visit's lasting effects. "I'll have to adopt a wait-and-see attitude," said Maynard. "... I'm hoping this isn't just a publicity stunt with a lot of rhetoric."

But one lasting effect was unmistakeable. Mrs. Carter painted a good-sized patch of the emergency room wall and displayed a sure technique with both brush and roller as well as with either hand. "Oh, I can paint with hoth hands," she said nonchalantly. "When Jimmy's term as governor was up I went back home almost every week and painted the entire house."

Later she visited the hospital's physical and occupational therapy ward and talked with a group of elderly patients, including 97-year-old Mary Newman, who was trying to learn to stand again for months in bed.

Mrs. Carter's visit to D.C. general grew out of a letter sent to her last year by Natalie D. Spingarn, a member of the hospital's board of governors. "It's like a magic want," said Spingarn of the visit. "Everybody's pitching in to help."

Dr. H. Donald Marshall II, director of D.C. general's emergency room, was also impressed, but he said the hospital needed more than beautification. "We're trying to change the type of physician who is working in the emergency room," said Marshall.

The First Lady rode Metrorail to and from D.C. general, accompanied by local and federal transportation officials. On the way there, Secretary of Transportation Brock Adams spoke proudly and at length of the subway system whose eventual cost (now estimated at close to $6 million) he has tried to hold down. "We're trying to get them to redesign it and save money," Adams told Mrs. Carter, "but it's a fine system."

Special arrangements had been made a both ends of Mrs. Carter's journey to assure that trains were ready and waiting. But no one, apparently, had ordered the Farecard machine at the McPherson Square station to accept the First Lady's money her side, Mrs. Carter repeatedly tried to insert her dollar in the slot, and was just as often repulsed. "There you go!" exclaimed Adams when the machine finally surrendered and gave her a Farecard.

"I like it very much," Mrs. Carter said at the end of her subway trip. It brings the city back to life."

She did her best to absolve the Farecard machine of all responisbility for her initial difficulties. The fault, said Mrs. Carter, was her own - for not taking the time to read the instructions first.