Previous reports on the different susceptibility to reserpine of responses acquired with positive and negative reinforcement have been inconsistent. Since none of the experiments has provided suitable controls for variables other than reinforcement, this experiment was designed to provide such controls and thus make possible a conclusive comparison. Both the positive (food) and negative (shock avoidance) situations shared the same type of response, stimulus characteristics, postinjection testing times, and response measure. Eight cats were trained to bar-press for a more appealing food than was supplied for their daily diet, and then were trained to press an adjacent bar to avoid shock. Tones were used as discriminative stimuli for both responses. One bar, mounted higher than the other, served as food bar for four 5s and as avoidance bar for four. The two tones were similarly counterbalanced. Half of each subgroup received reserpine and placebo before starting avoidance training but when tone discrimination was complete, as well as after completing all training. When each S reached criterion, 35 μgm/kg of reserpine was injected intramuscularly. Latency of each response was measured at 12 postinjection intervals, from 3 hr. to 124 hr. Fifteen days later, reserpine placebo was injected and measurements made at identical intervals. Each testing session consisted of five trials with each tone, in a mixed series. Finally, each cat was given a generalization test using a lone midway between the two training tones, and then both responses were extinguished. Derived scores were computed for each type of response for each S at each postinjection interval. First, the median for each session was converted to a difference score by subtracting the corresponding median of the preinjection session. These difference scores were then used in the formula (Avoidance-sub(Resperine) - Avoidance-sub(Placebo)) - (Foodesperine) - Food-sub(Placebo)), which gives a direct expression of the extent to which reserpine, as compared with placebo, slowed the shock response as compared with the food response. Lindquist's trend analysis applied to the group curve of derived scores showed that the shock response became significantly slower than the food response as the postinjection sessions proceeded and then gradually returned to normal. Intertrial responding on the shock bar increased significantly during reserpine but was unchanged on the food bar. Reserpine after food training alone had no effect on the response inhibition to the generalized tone. Neither the generalization nor the extinction data supported an hypothesis that the shock response was weaker than the food response. It is suggested that certain rhinencephalic structures are critically involved in both the performance of the shock response and the neurophysiological effect of reserpine. (PsycINFO Database Record (c) 2006 APA, all rights reserved)