Effects of d-amphetamine, chlordiazepoxide and promazine on responding of squirrel monkeys maintained under fixed-interval schedules of food presentation and stimulus-shock termination.

Responding of two squirrel monkeys was maintained under a multiple 5-min fixed-interval schedule of food presentation and termination of a stimulus in the presence of which shocks occurred. Under the stimulus-shock termination schedule, shocks occurred independently of responding, on the average of every three minutes; a response after 5 min terminated the prevailing stimulus and shock-presentation schedule. Response rates and patterns of responding under both schedules were comparable although they differed slightly between monkeys. d-Amphetamine increased and promazine decreased responding under both fixed-interval schedules. Chlordiazepoxide increased responding maintained by food presentation but decreased responding maintained by termination of the stimulus-shock complex. Under certain conditions and with certain drugs, the event that maintains responding can determine the effects a drug will have on behavior.