Former Foreign Minister Yohei Kono received part of his son's liver in a successful transplant operation that was completed early Wednesday.
The operation was performed at Shinshu University Hospital in Matsumoto, Nagano Prefecture.
The surgical team, headed by Seiji Kawasaki, started the operation on Kono, 65, and his eldest son, Taro, 39, at around 9:45 a.m. Tuesday.
Both Kono, who heads a small faction in the ruling Liberal Democratic Party, and his son, parliamentary secretary at the Public Management, Home Affairs, Posts and Telecommunications Ministry, are stable and have been moved to the intensive care unit, doctors said.
Taro was able to hold conversations, while the elder Kono opened his eyes in the morning, Kono's office said.
The former foreign minister was suffering from cirrhosis and hepatitis C and his condition had worsened, making the operation necessary.
Roughly a third of the son's liver was removed and transplanted. Because the liver is a regenerative organ, his liver will likely regain is normal size and functions in about one to two months, doctors said.
If all goes well, Taro will be able to leave the hospital in about two to three weeks, while his father may be discharged in about two months, his office and doctors said.
Kono's office released a statement prepared by the elder lawmaker prior to the surgery in which he expressed thanks to the surgical team and other staff at the hospital.
"I received many words of encouragement from people across the country, and I am sorry to be the cause of so much concern," he said in the statement. "Please forgive me for resting a while longer so that I can resume my political activities at the earliest time possible."
Doctors will continue to conduct tests to check for complications.