The electroencephalogram (EEG), electromyogram (EMG) and behavior were monitored in rats with chronic cerebrocortical and temporalis muscle electrodes. They were injected acutely with 0, 2, 4 and 8 mg/kg of phencyclidine (i.p.). The EEG power spectra correlated in a dose-related manner, with the direct EEG and behavioral effects of phencyclidine. At 2 mg/kg, the EEG consisted primarily of low amplitude theta activity from 6 to 8 Hz, accompanied by behavioral arousal and hyperactivity. At 4 mg/kg, the EEG theta activity persisted and was superimposed on a background of high amplitude slow waves which were associated with the emergence of ataxia. Locomotion and stereotypy intensified at this dose. The power spectra of these changes in the EEG were characterized by two distinct spectral peaks lying between 0-5 and 5-10 Hz. After 8 mg/kg, EEG activity shifted predominantly to lower frequencies between 2 and 3 Hz. This shift produced a large increase in spectral power in the 0-5 Hz band which was accompanied behaviorally by severe ataxia, limited locomotion, and stereotypy of head and forepaw movements. Similarities and differences between these EEG changes induced by phencyclidine and the EEG effects of ketamine and SKF 10047 were obtained.