Kuwaiti Crown Prince Sheikh Saad al-Abdullah al-Sabah has been admitted to hospital, official reports say.
The crown prince has made few public appearances in recent years
The agency said Sheikh Saad, who is in his 70s, has been suffering from a high level of blood glucose.
The crown prince has been deputising for the Emir of Kuwait, Sheikh Jaber al-Ahmad al-Sabah, who is in the US recovering from medical treatment.
Many of the daily duties of the ruler are already handled by Prime Minister Sheikh Sabah al-Ahmad al-Sabah.
In 1997, Sheikh Saad spent seven months abroad for treatment and recuperation from colonic bleeding.
He became crown prince in 1978, and was prime minister until July 2003, when the Emir appointed Sheik Sabah as prime minister, breaking with a political tradition that crown princes also serving as heads of government.
Like the emir, Sheik Saad has made few public appearances in recent years.