Soon after, there were reports that the kingdom has declared a state of alert with the Reuters news agnecy quoting a Saudi official as saying all leave of its security forces had been cancelled.
But the Saudi Interior Ministry spokesman, Mansour al-Turki, denied the report.
"This is absolutely not true," al-Turki said. "There's no cancelling of leaves and no state of emergency or anything."
Saudi official sources said King Fahd had been running a fever and had water in his lungs which required admitting him to hospital.
"King Fahd entered King Faisal Specialist Hospital in Riyadh this evening to undergo some medical examinations," SPA quoted a palace statement as saying. It gave no further details.
"We ask God to keep and care for the Custodian of the Two Holy Mosques and grant him health and wellbeing," the statement said.
But unconfirmed reports attributed to an unnamed official said the king had pneumonia and his condition was very serious.
News about the health condition of the king, who suffered a stroke in 1995, had led to a drop in Saudi Arabian stocks earlier this week.
The ruler of the world's top oil exporter has been ailing for the past decade, and rumours about his condition have frequently surfaced in Saudi Arabia and on international oil markets.
Day-to-day rule of the Gulf Arab state passed to the king's half-brother, Crown Prince Abd Allah, after his stroke.
Reports earlier in the week said the king was feeling tired but did not need admission into hospital.