(a) Mean (±SEM) number of PCP deliveries, (b) mean (±SEM) adjusted delay (seconds), and (c) mean (±SEM) percent choices made for the large, delayed reinforcer under FR 8 (filled symbols) and FR 32 (open symbols) as a function of PCP concentration (0.0625, 0.125, 0.25, and 0.5 mg/ml). Data represent the means of 5 sessions obtained in the group of 8 monkeys. *P < 0.05, ** P < 0.01.