Pigeons acquired a different four-response chain each session by responding sequentially on three keys in the presence of four colors. The response chain was maintained by food presentation under a fixed-ratio schedule. Errors produced a brief timeout but did not reset the chain. When phencyclidine was administered alone, the overall response rate decreased and the percent errors increased with increasing doses. Similar effects were found with a high dose of pentobarbital alone. When phencyclidine was administered in combination with pentobarbital, the phencyclidine dose-effect curves for rate and accuracy shifted to the left as the dose of pentobarbital was increased. Combinations of phencyclidine with a high dose of pentobarbital produced supra-additive effects; i.e., the effects on rate and accuracy were greater than expected from simple addition of the effects of each drug given alone. These results extend the generality of previous findings in patas monkeys in a similar repeated-acquisition task.