A new technique was applied to the study of operant discrimination in the Skinner box. The experimental session was divided into 20 periods of 2 min. each, with SD present during ten periods, and S[DELTA] for the other ten. Responses were reinforced aperiodically during the positive periods. Sixteen rats were run under these conditions for 27 days original discrimination; then 33 days on the reverse discrimination; then 5 days extinction. A control group of eight rats was given the same sequence of stimuli and reinforcement, but reinforcement was not correlated with either stimulus. The results were: 1. As the discrimination formed, the over-all rate of responding remained constant, and the responses shifted from the negative periods to the positive ones. This also held true during the learning of the reversed discrimination. 2. During extinction the discrimination was not abolished. Response rate dropped in the presence of both stimuli, but relatively more rapidly in the presence of the negative stimulus.