In urethane-anesthetized rats, a membrane potential oscillation (MPO) of up to 30 mV and 0.5-2 Hz (delta frequency range) was found in neurons of the nucleus accumbens. The membrane potential oscillations were of similar frequency and reversed in phase to the extracellular EEG of about 0.5 mV. In freely moving rats, a rhythmic delta EEG of 0.5-3 Hz was found in the nucleus accumbens, and it was of highest amplitude and regularity during awake immobility and face washing, less regular during slow wave sleep, and of the lowest amplitude during body and head movements and rapid eye movement sleep. The behavioural relation of the accumbens EEG was not critically affected after amphetamine, haloperidol, and parachlorophenylalanine, which depleted serotonin, although the accumbens EEG during awake immobility was less regular after blocking muscarinic cholinergic receptors by atropine sulfate. However, stimulation of the ventral tegmental area suppressed the accumbens delta membrane potential oscillations and EEG, and this effect was antagonized by haloperidol, suggesting that endogenous dopamine release may suppress the accumbens delta rhythm. It was concluded that the delta rhythm in the nucleus accumbens may represent a state of bilateral synchrony among accumbens neurons that is perhaps characteristic of an idling system, while desynchronization of the delta rhythm may occur closely with motor action.