"Proceedings were adjourned because he wasn't feeling well," said Nerma Jelacic, a spokeswoman for International Criminal Tribunal for the former Yugoslavia at The Hague, Netherlands.
Jelacic did not provide information about Mladic's condition, CNN reported.
Mladic, whose trial began in May, is charged with 11 counts of genocide, war crimes and crimes against humanity. Among the counts listed in the indictment against Mladic was the siege of Sarajevo, a 44-month campaign of sniping and shelling in which more than 10,000 people died.
Mladic avoided capture for nearly 16 years until May 2011 when police burst into the garden of a small house in northern Serbia.