In behaving rats, unit activity recorded from the ventral tegmentum and from the reticular formation was monitored during classical conditioning. Rewarding electric stimulation of the medial forebrain bundle was used as the unconditioned stimulus (UCS). Only those cells possessing prior responses to the conditioned stimulus (CS) changed their responses as a result of conditioning. Responses recorded from cells which were driven by both the auditory CS and the brain shock UCS were significantly more often changed than those driven by the CS alohe. These data show that the auditory and brain shock fields of influence interact in at least some brain regions prior to conditioning and that pairing the two kinds of stimuli is more likely to influence auditory responses recorded from these regions than those recorded elsewhere. It is possible to imagine that the intersection of the two fields is a sine qua non of conditioning and that the two prior actions caused the change by interacting at or near the recording point.