Hungry rats in Skinner boxes were trained to select the right or left lever as correct as a function of the presence of a tone or light stimulus, respectively. Correct responses produced liquid food rewards. Acute intraperitoneal administration of d-amphetamine or phenobarbital did not affect accuracy of performance, but decreased the percent trials attempted and lengthened response times as a function of increasing doses. The mean extra responses during the delay intervals generally decreased under phenobarbital and increased under d-amphetamine. Juvenile baboons were trained to respond on a delayed match-to-sample task in order to obtain banana pellet rewards. Acute intramuscular administration of phenobarbital produced a dose-related increase in errors, a decrease in mean extra responses and an increase in response times. A slight reduction in the percent trials attempted occurred only at the highest dose of the drug. Acute intramuscular d-amphetamine did not increase errors even at dose levels that increased reaction times, decreased extra responses and reduced the percent trials attempted.