The present study attempted to obtain information concerning the distribution of SP changes over monkey cortex in order to delineate the source and functional significance of contingent negative variation (CNV). Both shock-avoidance and food-reward paradigms were employed in an attempt to determine the relationship between slow potentials (SPs) and mode of reinforcement. Transcortical platinized-platinum electrodes were stereotaxically implanted in 5 rhesus monkeys. Subjects were trained to respond in order to either receive food reinforcement or avoid shock. A warning stimulus (click) was followed 1 sec later by the imperative stimulus (1500 c/sec tone of 1.5 sec duration); subjects were required to respond during the tone period by pressing a lever. Approximately 100 trials were given a day, 5 days a week, for a 2 month period. Brain electrical activity was suitably amplified and recorded on magnetic tape for subsequent computer analysis. Negative SP changes were recordable from many portions of frontal cortex. The most marked finding in the present study was the demonstration of the existence of two independent SPs related to the conditioning task. A frontal-dominant potential developed gradually with training, while the central-dominant SP was of significant magnitude from the onset of training. Each potential was of equal magnitude bilaterally and persisted for the duration of the 2 month testing period. Type of reinforcement was a significant variable in determining the distribution of cortical SPs. Shock reinforcement caused an increase in precentral regions that was not manifested in frontal areas. It was suggested that the frontal-dominant potential was analogous to CNV recorded in humans and reflected the association of the two stimuli, whereas the central-dominant SP was an indication of motor inhibition and readiness to respond. The possible relationship between these potentials was discussed in terms of triggering mechanisms for the initiation of timed movements. © 1973.