"I do not want to testify and refuse to testify for reasons of my health and that it would prejudice my own case," Mladic told the International Criminal Tribunal for the former Yugoslavia (ICTY), which he had previously labelled as "satanic".
Karadzic and Mladic both face charges over the massacre of almost 8,000 Muslim men and boys in the eastern town of Srebrenica in July 1995 as well as over the bloody 44-month siege of the Bosnian capital Sarajevo, which claimed an estimated 10,000 lives.
They could have been tried together had they been arrested around the same time. But Karadzic was arrested in July 2008 and Mladic in May 2011.
Karadzic, who is representing himself, asked Mladic whether he had ever informed him that civilians at Srebrenica would be killed, that terror through shelling or sniping would be used in Sarajevo or that Serb areas would be ethnically cleansed.
Mladic refused to answer all the questions on the same grounds. "You have confirmed my theory that the tribunal is not a court of law but a satanic court," Mladic said as he was led away.