Morphine, naloxone and nalorphine were studied for their effects on the performance of squirrel monkeys and pigeons responding under multiple fixed-interval (FI), fixed-ratio (FR) schedules of food presentation. Morphine generally produced only dose-related decreases in responding in both monkeys and pigeons; monkeys were 10 times more sensitive to morphine than pigeons. The only effect of lower doses of naloxone (0.01-1 mg/kg, monkeys; 1-10 mg/kg, pigeons) was to increase FI responding in some pigeons. Higher doses of naloxone (10-56 mg/kg), produced gross disturbances such as tremors and vomiting and decreased FI and FR responding of both monkeys and pigeons. Nalorphine had strikingly different effects on the behavior of the two species. In the pigeons, nalorphine consistently increased both FI and FR response rates at doses from 0.3 to 10 mg/kg and decreased responding only at doses of 30 to 100 mg/kg. Nalorphine did not increase responding at any dose in the monkeys and the pigeons, nalorphine was only one-tenth as potent as naloxone in antagonizing the effects of morphine on FI and FR responding. Decreasing response rates caused by nalorphine appeared to limit further its usefulness as a morphine antagonist. Antagonism of the rate-decreasing effects of morphine on FI and FR responding occurred over a narrower range of doses with nalorphine than with naloxone, especially in monkeys.