An Air Force C9 carrying Nancy Reagan slipped off the runway today at Charlie Brown Fulton County Airport west of Atlanta, the White House said. Mrs. Reagan was unhurt.

The first lady was buckled in her seat when an inboard wheel of the passenger jet, which was preparing to take off, slid off the pavement into the mud, said Wendy Weber, Mrs. Reagan's deputy press secretary.

Mrs. Reagan was "visibly shaken," said one escort official who asked to go unnamed. But no one was hurt.

"Is everyone all right?" the first lady asked immediately from her seat.

Mrs. Reagan called President Reagan from the Coca-Cola Lounge at the airport a little later, according to Weber.

"He asked 'Are you all right?' " according to Weber. "Then he said 'I'll be waiting for you.' "

Dobbins Air Force Base in nearby Marietta sent equipment to pull the plane out of the mud. Andrews Air Force Base sent another plane, an Air Force Gulfstream, to pick up the first lady and 15 others traveling with her, including her chief of staff Jack L. Courtemanche.

An airport official said the first lady's plane had made a sharp turn at the end of the runway and one wheel dropped off the paved surface and became stuck in the mud.

Weber said she had "no indication of cause" and did not know if there was the possibility of pilot error.

She said the accident occurred about 2:40 p.m. The first lady and others aboard deplaned via a movable staircase that was brought up to the plane and were taken to the lounge where Elaine Crispen, Mrs. Reagan's press secretary, telephoned both deputy press secretary Larry Speakes and Weber at the White House.

The first lady had flown to Atlanta in the morning to address a drug abuse conference where first ladies of six nations also were in attendance.

One of them, Italy's Anna Craxi, wife of Prime Minister Bettino Craxi, was a passenger at Mrs. Reagan's invitation. Mrs. Craxi, however, remained in Atlanta to attend a workshop arranged by PRIDE (Parents' Resource Institute for Drug Education), conference sponsors.

While an Air Force maintenance crew from Dobbins Air Force Base inspected the listing jet, the first lady sipped coffee with her staff in the elegant lounge, decorated with brass potted plants and burnished wood antiques. She read magazines, went over paperwork and flipped channels on the 25-inch color TV, searching for news.

"She was very anxious to find out what was going on in Washington," with the president's showdown over contra aid, said Peter Mathon, vice president of the Arthritis Foundation and a local volunteer who helped the White House press office with the first lady's visit.

"Her biggest concern was that everybody on the plane was safe."

In the lounge she chatted about the weather and the mudslides in California. There were no jokes about the plane, which was jacked up with airbags and taxied back onto the runway. After the plane sank into the mud, Dobbins Air Force Base got a phone call asking for a maintenance crew that was expert at jacking up airplanes, said a Dobbins public affairs officer. "They told us that the plane of the president's wife was stuck in the mud," he said, "so we are going to help if we can." By the time the plane was hauled out, the first lady was airborne on the Gulfstream jet.

It touched down here at 5:15, and within minutes a black Cadillac limousine had driven Mrs. Reagan the 200 yards from the lounge to her plane. She arrived at Andrews about an hour and a half later. High on a hill overlooking the runway, network cameramen and reporters were kept at bay by Fulton County police SWAT team.

"He [the pilot] just taxied out to the ramp, turned right and fell off the edge," said Cpl. J.D. Shirley, a SWAT team member who watched the mishap. He gazed down at a giant puddle from two days of rain. "We normally don't have that much rain. The ground is soft. It just sunk."

David Harrell, 32, a bemused carpenter fresh from a condominium job nearby, tugged at his cap, emblazoned with "I give a little extra," and said, "I just heard them talking about it on the radio, that Mrs. Reagan got stuck in the mud. I thought the deejay said she was a 'stick in the mud,' and I came on over. I guess she found out all about Georgia red clay."