This report describes periodic oscillations in electroencephalographic (EEG) and behavioral activity with a cycle length of 15–30 seconds in chair-restrained squirrel monkeys (Saimiri sciureus). These oscillations consisted of alternating episodes of vigilance, characterized by visual scanning and motor movement, and inattentiveness, characterized by behavioral quiescence with little eye or limb movement. During vigilance the EEG exhibited low-amplitude, high-frequency (> 16 Hz) activity. During quiescent periods, a high-amplitude synchronized EEG was present with activity in the 8–16-Hz band predominating. The presence or frequency of this EEG and behavioral periodicity was not modified by time of day, as no difference was found between morning and afternoon recording sessions. Although the factors or mechanisms responsible for this rhythm are unclear, it should be noted by those investigators studying the behavior or neurophysiology of Saimiri sciureus in the laboratory setting.