Saudi Arabia's King Abdullah will leave for the United States on Monday for further medical checks, the state news agency said on Sunday, after developing complications in a back condition.
"The king will leave on Monday for the United States to complete medical tests," the Saudi Press Agency said.
SPA also said that Crown Prince Sultan, who has had unspecified health problems over the past two years, would return to Saudi Arabia on Sunday evening from Morocco where he has been since August.
Both men are in their 80s.
Political stability in the monarchy is of global concern. The Gulf Arab state controls more than a fifth of the world's crude reserves, is a vital U.S. ally in the region, a major holder of dollar assets and home to the biggest Arab bourse.
Abdullah, thought to be 86 or 87, was admitted to hospital on Friday after a blood clot complicated a slipped disc he suffered the week before.
Prince Nayef, thought to be about 76, was appointed second deputy prime minister in 2009 in a move which analysts say could avert a power vacuum in the event of serious health problems afflicting the king and crown prince.
Last week the king transferred control of the National Guard, an elite Bedouin corps that handles domestic security, to his son Mitab.