The Imperial Household Agency says Emperor Akihito, 77, will be treated with medication. He underwent tests Friday at a Tokyo hospital after an earlier checkup detected insufficient blood flow to his heart during exercise.
The agency says Akihito will stay at the hospital overnight and then return to the palace, where he will continue his official duties.
The emperor underwent an operation for prostate cancer in 2003 and has been undergoing hormonal therapy to prevent its recurrence. He has scaled back his duties because of health reasons.