Rabbi Yosef Shalom Elyashiv, leader of the Lithuanian Orthodox public, has been hospitalized at the Shaare Zedek Medical Center after experiencing shortness of breath.
The rabbi, aged 101, has been suffering from ongoing weakness in recent months, and his condition deteriorated on Saturday night.
Elyashiv's personal physician was updated on the situation overnight and recommended that he be taken to a hospital.
Doctors decided to admit the rabbi after diagnosing pulmonary edema, his associates said, adding that he was conscious and in stable condition.
The associates reported that the rabbi's condition had improved on Sunday morning, enabling him to pray in his room. A decision on whether to discharge him will be made following another medical examination.
Many Ashkenazi Jews regard Elyashiv as the "posek ha-dor", the contemporary leading authority on Jewish Law, and are asking the public to pray for his recovery.