Three monkeys were each tested under a second-order fixed interval (FI) schedule with fixed-ratio (FR) components, a tandem FI FR schedule and an FI schedule while either food deprived or food satiated. Under the second-order schedule, every sixteenth lip-contact response on a drinking spout produced a brief stimulus and the first FR 16 begun and completed after the 60-min interval elapsed resulted in both the brief stimulus and access to orally delivered phencyclidine. The tandem schedule was similar, except that brief-stimulus presentations during the 60-min interval were omitted, and the third schedule was an FI 60 min. The reinforcer, orally delivered phencyclidine, was available only at the end of the 60-min session. The number of drug deliveries was either "limited" (300) or "unlimited" (for 1 hr) to determine whether increased responding due to food deprivation would occur in the absence of increased drug intake. Under all conditions response rates were nearly twice as high during food deprivation as they were during food satiation. The number of phencyclidine deliveries available at the end of the session has no systemic effect on the rate of pattern of responding, but quarter-life values were consistently lower during food deprivation than they were during food satiation. Under the tandem and FI schedules, overall response rates were much lower than under the second-order schedule, and quarter-life values were higher. When water was substituted for phencyclidine under each schedule condition, response rates and liquid deliveries generally declined to below phencyclidine levels indicating that the drug had been functioning as a reinforcer.(ABSTRACT TRUNCATED AT 250 WORDS)