The hippocampal theta rhythm has been extensively studied in many subprimate mammals. Considering the technical difficulties involved in recording from freely moving animals during voluntary motion and REM sleep, it was thought that urethane anesthesia might be appropriate for initial studies of the primate hippocampal EEG. Three of three macaques and one of two squirrel monkeys showed clear rhythmic hippocampal EEG activity. One very old squirrel monkey (a 16-year-old female) showed no theta activity in the hippocampal EEG. Similarities of the monkey theta activity with theta rhythm of urethane-anesthetized rats included: (1) a high coherence between recordings from electrodes separated by several millimeters within the hippocampal formation; (2) sensitivity of the theta activity to muscarinic drugs; and (3) its correlation with spontaneous movements during light anesthesia. Important differences were: (1) the frequency of the monkey theta activity was 7-9 Hz compared to the 4-5 Hz found in rats; (2) theta activity was not detected in the distal apical dendritic regions of CA1 or dentate in the monkey; (3) considerable amounts of low-frequency EEG co-existed with the monkey theta activity; and (4) the durations of bouts of theta activity in monkeys were much shorter than in rats. We conclude that primates generate hippocampal theta activity homologous, but not identical, to that of rats.