Behavior of squirrel monkeys, maintained by the termination of stimuli associated with electric shock, was suppressed by response-dependent shock delivery. The effects of pentobarbital on this behavior depended on whether monkeys had previously received morphine. In monkeys without experience with drugs, pentobarbital increased responding. In monkeys with recent experience with morphine, however, pentobarbital resulted in a smaller increase or decrease in responding. The rate-decreasing effects of pentobarbital after a history of morphine administration could be reversed by the administration of d-amphetamine. These findings suggest that the behavioral effects of abused drugs may depend on previous experience with other drugs, even when those drugs are from a different pharmacological class.