Rats performing on a progressive-ratio schedule that required five additional responses for each successive food reinforcement were administered d-amphetamine. The number of responses required increased until the next ratio was not completed within 15 min. The number of reinforcements obtained during a session increased with increasing doses of d-amphetamine from 0.25 mg/kg to a maximum behavioral effect at 1.0 to 2.0 mg/kg followed by a decline with higher doses. To assess the effects of hyperbaric air on the established d-amphetamine performance, the same doses were administered to rats breathing compressed air at seven times normal atmospheric pressure (7.1 ATA). In the hyperbaric condition the dose-effect function was displaced toward smaller doses such that the maximum behavioral effect was obtained at lower doses than under normal atmospheric pressure; does that produced maximum behavioral effects under normal pressure produced a decline in reinforcements obtained.