Electrophysiological parameters of spontaneous cellular activity in the nucleus accumbens septi (NAS) and responses evoked by afferent (fimbria) stimulation were characterized in halothane-anesthetized rats. A heterogeneous pattern of cell activity and single cell responses to stimulation were observed throughout the NAS. Stimulation-evoked discharges from inactive cells regularly coincided with evoked 'field' responses. Because the NAS is critical for opiate drug self-administration in rats, the effects of systemic heroin and morphine were then observed. These drugs also had mixed effects on the spontaneous activity of NAS cells encountered. While most cells were depressed, other cells were excited or were unaffected. Opiates had no effect on stimulation-evoked activity. These data lend neurophysiological and neuropharmacological support to the neuroanatomical observations of cellular heterogeneity within this structure.