Despite intensive studies on cerebral activity during trances involving tranquil arousal states, there are little data on physiological basis of naturally induced possession trances involving hyperarousal active states because of the difficulty of gathering data from participants within a natural cultural context in the field. We investigated the characteristics of electroencephalograms (EEGs) that were specific for naturally induced possession trances involving hyperarousal states in actual rituals. We measured the EEG signals of 12 healthy participants, seven with trance and five without trance, before, during, and after a dedicatory ritual drama in Bali, Indonesia, using a custom-modified field telemetry system. During trance, θ (4–7.5 Hz), α-1 (8–9.5 Hz), α-2 (10–12.5 Hz), and β (13–30 Hz) signals were significantly increased compared with those during the control phases. Such findings were not observed in participants without trance when they performed similar movements in the rituals. The α-1 and α-2 signals tended to remain elevated for several minutes postritual compared with those recorded during the preritual resting state. These results suggest that spontaneous EEG patterns during possession trances may be related, at least in part, to the activation of the reward-generating neuronal system situated in deep-lying brain structures and deactivation of the cerebral cortex.