Five adult, male White Carneaux pigeons were trained to respond under a titrating delayed matching-to-sample schedule of reinforcement. Under this schedule, the length of the delay was changed as a function of the pigeons' performances in a manner that resulted in approximately 80% accuracy in matching performance. During the first five trials of each session, the delay value was fixed at either 3 sec or 30 sec (starting delay). On the 6th and all subsequent trials, the length of the delay value was increased by a fixed amount (the titration step), did not change or was decreased by an amount equal to the titration step. Initially, the starting delay and the titration step were set at 3 sec. These values were then systematically changed to a starting delay of 30 and a titration step of 3, and finally, a starting delay of 3 and a titration step of 1. Using this procedure, the effects of pentobarbital (0.3-10 mg/kg), cocaine (0.01-10 mg/kg), d-amphetamine (0.003-3 mg/kg) and phencyclidine (0.03-3 mg/kg) were determined under the different titration parameters. Pentobarbital was shown to decrease matching performance at doses (3, 5.6 and 10 mg/kg) that did not decrease response rates. Phencyclidine produced a similar effect at a dose of 1 mg/kg under two of the three phases of the experiment. No such specific effects were observed with cocaine or d-amphetamine. Thus, of the four drugs studied, pentobarbital showed the most pronounced effect on matching performance. Whether this represents merely a loss of stimulus control or a specific effect on memory is yet to be determined. In addition, the results of these experiments clearly show that when the delay value is allowed to change as a function of the pigeons' performance, pigeons are able to perform at or above 80% accuracy at longer delay values than those generally reported using fixed-delay values.