In order to assess the role of histamine H3 receptors in the discriminative-stimulus effects of methamphetamine, rats were trained to discriminate 1.0 mg/kg methamphetamine, i.p., from saline under a fixed-ratio schedule of food presentation. The histamine H3 receptor antagonist thioperamide (1.0 mg/kg s.c.), which facilitates histamine release, significantly shifted the methamphetamine dose-response curve to the left when tested together with different doses of methamphetamine and markedly extended the time-course of methamphetamine's discriminative-stimulus effects. The histamine H3 receptor agonist R-alpha-methylhistamine (3.0 mg/kg i.p.), which blocks histamine release, did not produce any effects when given alone, but it attenuated the effects of thioperamide on the methamphetamine dose-response curve when both drugs were given together. Thus, methamphetamine's discriminative-stimulus effects are markedly potentiated by the blockade of histamine H3 receptors by thioperamide. This is likely due to thioperamide's actions at histamine H3 autoreceptors on histaminergic neurons to facilitate release of histamine by methamphetamine or at histamine H3 heteroreceptors on other monoaminergic neurons (e.g., dopaminergic, serotonergic or noradrenergic) to facilitate release of other neurotransmitters.