The Duke of Edinburgh has been hospitalised as a "precaution", according to Buckingham Palace.
Prince Philip was taken to Aberdeen Royal Infirmary on Wednesday afternoon, although as yet there is no confirmed reason for the admission.
A Buckingham Palace spokesman said Philip travelled to the hospital, about 40 miles away from Balmoral, by road.
The Duke went to the Scottish royal residence after attending a number of Olympic events, including granddaughter Zara Phillips' Olympic debut on July 29.
The spokesman added: "He has been resident at the castle for a short while, but whilst he has been there, he has also undertaken engagements on the Isle of Wight."
The Duke appeared to be in good spirits as he took part in events during Cowes Week, in his role as Admiral and member of the Royal Yacht Squadron.
The royal has been increasingly unwell over the past few months, most recently he was hospitalised for several days with a bladder infection shortly after the Diamond Jubilee river pageant.
The 91-year-old Duke also missed Christmas with the Royal family at the Sandringham Estate in Norfolk following an operation for a blocked coronary artery.