Saudi Arabia's King Fahd is being treated in hospital amid conflicting reports about his condition.
King Fahd has been Saudi Arabia's monarch since 1982
The authorities say he is in good health after being admitted for tests in the capital, Riyadh, on Friday.
But some medical sources suggest the king is being treated for possible pneumonia following a bout of flu.
The BBC's Frank Gardner says there have been rumours about King Fahd's health before but the latest scare could be more serious.
In 1995 he suffered a stroke, and Crown Prince Abdullah has performed most of his functions since then.
Prince Abdullah is regarded as successor to the throne.
Opposition sources in London predict that if the king dies there may well be a power struggle against the ruling princes.
There is heavy security around Riyadh's King Faisal hospital. Saudi security sources said a number of princes from the ruling family have been arriving in the capital.
The royal palace has not given details of the king's symptoms.
On Saturday Reuters news agency quoted officials as saying he was stable following treatment to drain water from his lungs.
The health scare came as leaders from the Gulf Cooperation Council were preparing to meet in Riyadh on Saturday.
Saudi TV showed Prince Abdullah speaking to leaders from Qatar, Kuwait, and the United Arab Emirates, Reuters reported.
King Fahd, who is in his early 80s, has been little more than a titular leader since suffering his stroke.
However, he has remained an important figure in the region and the ultimate arbiter of Saudi policy.
Saudi Arabia is the world's largest oil producer and has emerged from being an underdeveloped desert kingdom to become one of the wealthiest nations in the region.
On Wednesday, rumours of his failing health sent the value of Saudi stocks tumbling by 5%.