Rats were trained to discriminate d-amphetamine (1.0 mg/kg) or clonidine (0.06 mg/kg) from saline in a standard, two-lever procedure with food reinforcement (n = 6). The similarity between the discriminable properties of amphetamine and clonidine was both partial and asymmetrical. Cross tests with amylobarbitone and chlordiazepoxide in rats trained with clonidine suggested that a major component of the clonidine stimulus may be general sedation. Pretreatment with the alpha 2-adrenoreceptor antagonist, piperoxane partially antagonized the discriminative stimulus produced by clonidine. The alpha 1 antagonist, phenoxybenzamine failed to alter the clonidine stimulus. Although amphetamine and clonidine may share some elements in common, this may not represent a noradrenergic component in the amphetamine discriminative stimulus since noradrenergic mediation of the clonidine stimulus was not established.