Single unit recording in rat nucleus accumbens (NAcc) was used to ascertain NAcc neuronal activity in mediating of reward including its anticipation. Of the 103 neurons investigated, 63% showed some response in connection with the task activity. Of these, 20 units responded during delivery of the primary reward (food and/or water) and five responded during the time period preceding reward if the reward was delayed (four to food, one to water). These result suggest that NAcc neurons responded not only to the delivery of primary reward and task inducing anticipation of reward, but also represent the difference of reward quality between food and water specifically.