Rate dependent drug effects: possible state dependency.

Of 6 rats given extensive experience responding on a fixed-interval reinforcement schedule after injections of 1 mg/kg d-amphetamine, 3 showed significant rate dependent changes in responding when presession saline was substituted for d-amphetamine. Low rates were increased and high rates decreased by the change from d-amphetamine to saline, a result commonly found when d-amphetamine is first introduced. This indicates that rate dependent changes in fixed-interval responding may be state dependent phenomena.