Japan's ageing Emperor Akihito underwent a heart examination at a hospital on Saturday after showing symptoms of restricted blood flow to his heart, amid increasing worries about his health.
The 78-year-old monarch, accompanied by Empress Michiko, was admitted to the University of Tokyo Hospital in the morning and had an hour-long scan of his coronary arteries at midday, the Imperial Household Agency said.
He was scheduled to stay at the hospital overnight for observation.
The test was arranged after periodic electrocardiograms showed restricted blood flow to his heart, the agency said.
He also had an angiogram last year at the same hospital, which found his arteries were narrowing and he was put on medication.
In November, the emperor spent 19 days in hospital suffering mild pneumonia.
Akihito, who ascended the throne in 1989 following the death of his father Hirohito, underwent surgery for prostate cancer in 2003.
The emperor's second son Prince Akishino in November suggested there should be a discussion about setting a retirement age for the titular head of state.
Despite being stripped of much of its mystique and its quasi-divine status in the aftermath of World War II, the Japanese throne is held in deep respect by much of the public.