Abstract Squirrel monkeys were given extensive discrimination training in which a lever response following one tone (S d ) was positively reinforced by a pellet of food and a response following the second tone (S Δ ) was negatively reinforced with electric shock. Time histograms of single unit activity were averaged over ten S d and ten S Δ presentations. Analyses of unit discharge patterns were made during spontaneous control periods, stimulus periods, and poststimulus periods. Of the structures sampled (amygdala, corona radiata, cortex, globus pallidus, internal capsule and putamen) the globus pallidus yielded the highest percentage of differentiated changes in unit activity during conditioned discrimination. The greatest changes in discharge frequency of pallidal units were associated with S d trials with few significant responses to S Δ . The unitary response to S d appeared to consist of two distinct and independent components: facilitatory, inhibitory or complex graded unit response during the presentation of S d and a pronounced period of inhibition during post S d periods. We tentatively suggest that conditioned unitary changes associated with the positive discriminative cue reflect changes in attention, set or anticipation while the post S d inhibitory phenomena are related to voluntary motor sequences leading to and including the consumption of food rewards.