The sources of input and the behavioral effects of lesions and drug administration suggest that the striatum participates in motivational processes. We investigated the activity of single striatal neurons of monkeys in response to reward delivered for performing in a go-nogo task. A drop of liquid was given each time the animal correctly executed or withheld an arm movement in reaction to a visual stimulus. Of 1593 neurons, 115 showed increased activity in response to delivery of liquid reward in both go and nogo trials. Responding neurons were predominantly located in dorsal and ventromedial parts of anterior putamen, in dorsal and ventral caudate, and in nucleus accumbens. They were twice as frequent in ventral as compared to dorsal striatal areas. Responses occurred at a median latency of 337 ms and lasted for 525 ms, with insignificant differences between dorsal and ventral striatum. Reward responses differed from activity recorded in the face area of posterior putamen which varied synchronously with individual mouth movements. Responses were directly related to delivery of primary liquid reward and not to auditory stimuli associated with it. Most of them also occurred when reward was delivered outside of the task. These results demonstrate that neurons of dorsal and particularly ventral striatum are involved in processing information concerning the attribution of primary reward.