Twelve rhesus monkeys were studied under a fixed-ratio (FR) schedule of intravenous procaine or d-amphetamine injection from 8 A.M. to 4 P.M. daily. Under the FR schedule, every nth lever press produced an injection. The FR value (n) and the dose per injection of procaine and d-amphetamine were varied systematically. At a FR value of 10, responding was maintained by doses of procaine ranging from 0.125 to 12 mg/kg/injection and by doses of d-amphetamine ranging from 0.01 to 0.1 mg/kg/injection. At doses of 1 mg/kg/injection of procaine and 0.1 mg/kg/injection of d-amphetamine, responding was maintained at FR values up to 100 by procaine and d-amphetamine but not by saline. Responding and drug intake were relatively constant throughout each 8-hour session with procaine, but responding tended to decrease and was more variable over the session with d-amphetamine. No toxic effects were observed in doses up to 6 mg/kg/injection with procaine. At this dose, eating and drinking ceased during the period of access to the drug. One of the four monkeys died at 8 mg/kg/injection of procaine. At 12 mg/kg/injection all three monkeys tested showed signs of toxicity.