The preliminary medical reports on the president's condition were "very positive," according to White House press secretary Marlin Fitzwater. He said there was no immediate indication that Mr. Bush had suffered a heart attack.
An initial diagnosis was that Mr. Bush, 66, had experienced an episode of atrial fibrillation, or irregular heartbeat, that lasted more than 90 minutes. He was given the drug digoxin, or digitalis, which is commonly administered to heart patients to restore heart rhythm.
The president was being kept overnight for observation at the hospital, where he arrived by helicopter, accompanied by his wife, Barbara, shortly before 6 p.m.
In a nationally televised statement about 2 1/2 hours later, Mr. Fitzwater described Mr. Bush as "relaxed, comfortable . . . and in good spirits."
The White House spokesman, clearly attempting to allay any fears about the president's condition, said Mr. Bush hoped to return to Camp David today and had not canceled any of his schedule for this week.
He said that no steps had been taken to transfer presidential power temporarily to Vice President Dan Quayle because "this was never considered to be that serious. There was never any question of the president losing consciousness and being unable to continue his functions."
"He could be back jogging in a matter of days," said Mr. Fitzwater, quoting the president's doctors as saying that an irregular heartbeat of the sort Mr. Bush experienced is not the sort that ordinarily limits physical activity.
The president had official paperwork brought to his hospital room, where he and Mrs. Bush dined yesterday evening on steak, salad, mixed vegetables and skim milk.
As if to underscore his spokesman's upbeat remarks, Mr. Bush, who apparently was watching the press briefing on television tTC from his hospital room, relayed an answer to a reporter's question about how long he had been exercising at the time the episode occurred.
Mr. Bush, whose travel pace exceeds that of any other recent president, left Washington early yesterday for a college commencement address in Michigan. He returned to Andrews Air Force Base in suburban Maryland about 1:45 p.m. and boarded a Marine helicopter for the 30-minute flight to Camp David.
Reporters who accompanied Mr. Bush on the trip to Michigan, where he delivered a 20-minute speech attacking the campus phenomenon of "political correctness," said there was nothing unusual about his physical appearance.
Shortly before 4 p.m., Mr. Bush, a devoted jogger, went out for a run, accompanied by Secret Service guards. About 4:20 p.m., or after about 35 minutes of jogging and walking, according to Mr. Bush's own estimate, he complained of shortness of breath.
A White House physician examined Mr. Bush and recommended that he be taken to Bethesda for further treatment and observation.
When Mr. Bush arrived at the hospital, at 5:58 p.m., the irregular heartbeat was continuing, but doctors found his condition to be stable with no other symptoms, the White House said. The president walked from his helicopter to a car that took him a few hundred feet into the hospital building.
Mr. Fitzwater said he did not know whether the cardiac irregularity was continuing or whether it had been brought under control by the drug Mr. Bush was given by his physicians, who included Dr. Michael Nash, of the White House medical unit, and Dr. John A. Williams III, a Navy cardiologist on the hospital staff.
Mr. Bush, who had a detailed physical checkup at Bethesda in March, had no warning or earlier signs of medical problems.
White House Chief of Staff John H. Sununu joined Mr. Bush at the hospital. Mr. Quayle, who remained at his official residence in Northwest Washington, phoned the president at 7:20 p.m. and reported that Mr. Bush was "in excellent spirits," the White House said.
Mr. Bush's staff sought to play down the seriousness of the event, and doctors with knowledge of atrial fibrillation said it was quite possible that he would suffer no long-term problems as a result of the incident.
"The president's in good spirits, and we don't have any undue alarm," said Mr. Fitzwater, who retreated to his West Wing office following his televised briefing to huddle with other top White House aides, including Deputy Chief of Staff Andrew Card and Counsel C. Boyden Gray.
Mr. Fitzwater, asked whether an anxiety attack might have caused Mr. Bush's heart to begin beating wildly, responded, "The president is calm, cool and collected and never anxious in his life. Well, maybe once, but I can't remember it."
However, the episode, the first serious medical problem in Mr. Bush's 28 months in office, immediately focused attention on Mr. Quayle, who would assume the duties of the presidency if Mr. Bush were to become incapacitated. Public opinion polls have repeatedly shown that a majority of Americans have serious doubts about Mr. Quayle's ability to carry out those duties, and questions have been raised in political circles about whether he would be dumped in 1992, a notion that Mr. Bush and his top aides have heatedly rejected.
Mr. Fitzwater said Mr. Sununu had telephoned Mr. Quayle and National Security Adviser Brent Scowcroft before Mr. Bush left Camp David to inform them of the medical emergency. But the spokesman said no steps were taken to implement any of the procedures prescribed by law in the event of presidential incapacity.
Mr. Bush, who was reported by his aides to be making jokes with his doctors about returning to his interrupted jog, was to remain at the hospital at least until today, when the results of tests conducted yesterday evening would be available, the White House said.
He is expected to continue taking digoxin, an oral medication with no serious side effects, for at least the next few weeks, his spokesman said. At that point, Mr. Bush's condition would be re-evaluated.
"I'm feeling great," Mr. Bush was quoted as telling Mr. Fitzwater in a phone call after the briefing. "Hope to go back to Camp David tomorrow [Sunday]."
Mr. Fitzwater, after speaking with the president, told reporters: "He's in great spirits, thinks it's a lot of hoopla about nothing."
Mr. Bush's predecessor, Ronald Reagan, was the last president to be rushed to the hospital, following an assassination attempt in March 1981. At the time, the White House initially played down the seriousness of the shooting, which nearly took Mr. Reagan's life.
In 1988, when Mr. Reagan underwent surgery for colon cancer, the terms of the 25th Amendment were invoked, making then-Vice President Bush the acting president for the eight hours that Mr. Reagan was under anesthesia.
Mr. Bush, who turns 67 next month, went through a vigorous workout at the White House only last Wednesday at an event designed to dramatize the health benefits of regular exercise.
"No matter how old you are or what kind of shape you're in, exercise helps every one of us live longer, healthier, more enjoyable lives," said Mr. Bush, who plays tennis, "aerobic golf" and other sports in addition to jogging.