The 83-year-old was undergoing medical tests at Rome's San Giovanni hospital after being admitted late Monday (January 22) with heart fibrillation, a type of irregular heartbeat, his son said.
Zeffirelli requested an ambulance while taking part in the recording of a TV show, according to the ANSA news agency. He will probably be released over the next few days.
The most recent opera Zeffirelli staged was Verdi's Aida, which opened the 2006-07 season at La Scala. (The production subsequently drew international attention for other reasons.) He is reportedly working on the sets for La traviata, which stars Angela Gheorghiu and Roberto Alagna and opens at the Opera di Roma on April 20, according to ANSA.
Zeffirelli has staged numerous works for the Metropolitan Opera, including Traviata, Carmen, Tosca and La bohme. He received an Oscar nomination for the 1968 film version of Romeo and Juliet and acclaim for the 1977 television miniseries Jesus of Nazareth.