Holmes was arraigned Tuesday morning in Arapahoe County Court on 166 counts that included murder, attempted murder and other crimes related to the deadly attack last July during a midnight showing of "The Dark Knight Rises" in an Aurora, Colo., movie theater. Twelve people were killed and 58 were wounded.
The plea was entered over the objections of Holmes' attorneys, who said they weren't ready to respond to the charges, The Denver Post reported. The objections open the door for Holmes to later change his plea to not guilty by reason of insanity, the newspaper said.
If he does change his plea, Holmes would be required to agree to a psychiatric evaluation, during which he could be drugged to aid in the examination, the report said.
Eighteenth District Chief Judge William Sylvester previously rejected a challenge by Holmes' attorneys to rule parts of the state's insanity defense laws unconstitutional. The attorneys argued the requirement that a defendant cooperate with examining psychiatrists is a violation of the defendant's privilege against compelled self-incrimination.