Pigeons prefer an alternative that provides them with a stimulus 20% of the time that predicts 10 pellets of food and a different stimulus 80% of the time that predicts 0 pellets, over an alternative that provides them with a stimulus that always predicts 3 pellets of food, even though the preferred alternative provides them with considerably less food. It appears that the stimulus that predicts 10 pellets acts as a strong conditioned reinforcer, despite the fact that the stimulus that predicts 0 pellets occurs 4 times as often. In the present research, we tested the hypothesis that early in training conditioned inhibition develops to the 0-pellet stimulus, but later in training it dissipates. We trained pigeons with a hue as the 10-pellet stimulus and a vertical line as the 0-pellet stimulus. To assess the inhibitory value of the vertical line, we compared responding to the 10-pellet hue with responding to the compound of the 10-pellet hue and the vertical line early in training and once again late in training, using both a within-subject design (Experiment 1) and a between-groups design (Experiment 2). We found that there was a significant reduction in inhibition between the early test (when pigeons chose optimally) and late test (when choice was suboptimal). Thus, the increase in suboptimal choice may result from the decline in inhibition to the 0-pellet stimulus. Implications for human gambling behavior are considered.