Neuronal activity in the dorsolateral and ventral prefrontal cortex was studied during the learning by six monkeys of the performance of a two-color visual discrimination task. The GO/NO-GO task or its reversal consisted of five periods: the Start Lamp Period, Lamp-off Period, Cue Period, Response Period, and ITI Period (Reward and Control Periods). In sum, 271 task-related neurons were studied by means of 545 penetrations. Discharge rates increased or decreased from the rate of ITI or preceding periods of the task. Discharge rate changes were of either the phasic or tonic pattern. During motor learning extending from a poor performance level (50-63% correct), through intermediate (60-83%), to well-trained levels (above 85%), the number of task-related neurons increased in the Cue, Response and ITI Periods, and more task-related neurons showed changes in more than one period. During the learning process, not only do more prefrontal neurons respond phasically to visual stimuli in the Cue and Response Periods or to reward or trial end in ITI, and become activated before lever release, but also they become active tonically in the Cue, Response and ITI Periods.