The effects of 0.75 mg/kg of R-DOM and 1.0 mg/kg of S-amphetamine served as discriminative stimuli in the control of a two-lever discrimination response. Dose-response evaluation of the two agents indicated that in S-amphetamine-trained animals the drug effects produced by test doses of 0.5 to 2.2 mg/kg of S-amphetamine could be considered as a dose-dependent continuum of low to high stimulus intensity. In R-DOM trained subjects, low to moderate doses of R-DOM (0.5-1.5 mg/kg) produced dose-related discriminative responding, but the high dose of R-DOM (2.2 mg/kg) resulted in random responding. When animals trained to discriminate 1.0 mg/kg of S-amphetamine from saline were given 0.5 or 0.75 mg/kg of R-DOM, they tended to respond on the drug correct lever. However, higher doses of R-DOM (1.5 and 2.2 mg/kg) produced random responding. Similarly, animals trained to discriminate 0.75 mg/kg of R-DOM from saline exhibited correct responses following 0.5 and 0.75 mg/kg of S-amphetamine, but not after 1.5 and 2.2 mg/kg. These data suggest that low doses of R-DOM produce discriminative stimuli that are qualitatively different from those of higher doses. Stimuli following low doses of R-DOM also appear similar in some respects to those following low doses of S-amphetamine.