The Ss received either 0 or 50% postcriterial trials on a verbal discrimination task. Following original training 25, 50, 75, or 100% of the items received a reversed reinforcement contingency. Main effects of degree of training and of percentage of items reversed were significant. The speed with which 100% reversal was obtained suggested that Ss utilized a concept of reversal" in this task. © 1969 Academic Press Inc. All rights reserved."