A spokeswoman for Berlin's Staatsoper told the AP that the 63-year-old conductor, who is music director of the Staatsoper and the Chicago Symphony Orchestra, "just wasn't feeling well at all." He is still under doctors' observation.
Conductor Julien Salemkour substituted for Barenboim at a special concert marking Mozart's 250th birthday at the Staatskapelle on Friday.
Barenboim is due to begin a U.S. tour with the Staatskapelle on February 3; it is unclear whether it will be rescheduled. The spokeswoman said, "The U.S.A. tour is not yet canceled, we're waiting on the examinations, which are ongoing."