Rats were trained to discriminate between the interoceptive cues produced by 6.0 mg/kg pentobarbital and 0.8 mg/kg d-amphetamine, both administered intraperitoneally, using a two-lever drug discrimination task. Once trained, intraperitoneal administration of 200 mg/kg ethanol produced pentobarbital-like lever selection at 5 and 15 min post-injection with a return to random levels at 30, 45 and 60 min post-administration. In contrast, administration of 100 mg/kg ethanol produced a significantly greater number of amphetamine-appropriate lever selections at 15 min post-injection. Time-course experiments using 6.0 mg/kg pentobarbital indicated the production of predominantly pentobarbital lever selection for 60 min with a return to random responding (50% on each of the two levers) at 90 min post-administration. These results indicate that a low dose of ethanol can produce more amphetamine-like subjective cues than a higher dose and this effect is time-dependent. Neurochemical evidence is cited to illustrate the biphasic effect of ethanol as it is seen in animals and man.