Pigeons learned to discriminate between a white vertical line on a dark background (S+) and a monochromatic circle of light (S—) either with or without responses to S— (errors). Gradients of inhibition, which were centered around S—, and which had greater than zero slopes, were obtained only from those subjects who learned to discriminate with errors. The results indicate that the occurrence of errors is a necessary condition for S— to function as an inhibitory stimulus. This finding is consistent with other performance differences in subjects who have learned to discriminate with and without errors.