The firing patterns of nucleus accumbens (NA) neurons in the rat were recorded during cocaine self-administration and responding for water. Recordings were obtained from permanently implanted multiple-electrode arrays (eight microwires) inserted bilaterally into rostral portions of the NA in subjects (n = 18) exhibiting stable cocaine self- administration (0.33 mg/infusion), and during stable responding for water reinforcement. Electronically isolated and identified NA neurons exhibited four distinct patterns of phasic activity relative to the reinforced response. Three of these firing patterns were observed during both cocaine self-administration and water reinforcement sessions. Response-related activity was categorized by cells that showed an anticipatory increase in firing rate during the preresponse phase (type PR), and by cells that were excited (type RFE) or inhibited (type RFI) following the response in the reinforcement phase. PR and RFE cells showed significantly reduced peak firing during cocaine self- administration, compared to similar cells in water reinforcement sessions. A fourth type of NA firing pattern (type PR+RF) was observed only in cells recorded during cocaine self-administration sessions (Carelli et al., 1993b). PR+RF neurons exhibited two distinct peaks, one preceding the response and terminating at response completion (like PR cells), and a second peak immediately following the response (like RFE cells) with an inhibitory period between the two peaks (like RFI cells). The findings are discussed in terms of the role of the NA in mediating the reinforcing properties of both cocaine and water.