Emperor Akihito, who underwent heart surgery on Saturday, is recovering well and was able to drink water on Sunday morning in the intensive care unit where he is being treated, Imperial Household Agency officials said.
The agency said the Emperor asked a doctor when he can start reading books.
Empress Michiko visited University of Tokyo Hospital to see her 78-year-old husband, while many people signed books for well-wishers in front of the Imperial Palace in central Tokyo.
Crown Prince Naruhito and Prince Akishino visited their father on Sunday afternoon.
Asked about the Emperor's heart bypass surgery, Prime Minister Yoshihiko Noda told reporters at his official residence, "I'm relieved that it ended without complications. I want to pray for his quick recovery."
The Emperor underwent coronary artery bypass surgery on Saturday. The operation, which lasted about four hours, ended without complications or significant bleeding, according to a team of physicians from the hospital and Juntendo University Hospital who operated on him.