The effects of repeated methamphetamine (4.0 mg/kg) plus scopolamine (0.5 mg/kg) treatment on behavioral sensitization and drug conditioning in rats were compared with the effects of repeated methamphetamine treatment. Behavioral sensitization induced by repeated methamphetamine plus scopolamine treatment was more vigorous than that induced by repeated methamphetamine treatment. Repeated methamphetamine plus scopolamine treatment produced sensitized responses, not only to methamphetamine plus scopolamine and methamphetamine but also, to a lesser extent, to scopolamine. Methamphetamine plus scopolamine-sensitized rats but not methamphetamine-sensitized rats exhibited conditioned responses to a low-frequency tone (300 Hz, 100 dB) associated with the drug state, suggesting that robust methamphetamine plus scopolamine-induced behavioral sensitization may lead to enhanced conditioning. It is plausible that robust behavioral sensitization might operate via a reciprocal balance between the dopaminergic and cholinergic systems in favor of dopaminergic dominance. Conditioning to the drug-associated tone may be mediated via a reciprocal balance between the two transmitter systems.