Effects of iontophoretically applied dopamine were investigated in prefrontal neurons of the monkey which showed activity changes during a visual reaction time task. The task consisted of an initial waiting phase (3.0 s), a warning phase (green lamp, 1.5-3.5 s), a lever-release GO phase (red lamp) and a reward phase. The neurons (n = 99) showed their activity changes during the warming (n = 31), GO (n = 43) or reward (n = 25) phase. Among those, dopamine predominantly influenced the GO phase-related activities (39/43) and the activity changes were enhanced by dopamine. Further, fluphenazine attenuated the GO phase-related activity changes. Results suggest that prefrontal dopamine may be involved in modulations of neuronal activities related to motor performance.