When rats and mice are free to explore a familiar environment they spend more time in a previously rewarded location. This conditioned place preference (CPP) results from an increased probability of initiating transitions from an unrewarded location to one previously paired with reward. We recorded nucleus accumbens (NAc) neurons while rats explored a three-room in-line apparatus. Before place conditioning, approximately equal proportions of NAc neurons show excitations or inhibitions when the rat is in each of the rooms (morphine paired, center or saline paired). Conditioning increased the proportion of neurons inhibited while the rat was in the morphine room and neurons excited in the saline room. Many of the neurons in these two groups responded during room transitions. Furthermore, the postconditioning increase in the population of neurons with room-selective responding persisted for several weeks after the last morphine treatment. This long-lasting change in population responses of NAc neurons to initially neutral locations is a neural correlate of the change in location preference manifest as CPP.