We investigated the effects of intracerebroventricular injection of substance P (SP) on the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance in the mouse. SP (0.001-3 micrograms) alone did not influence either spontaneous alternation performance or total arm entries. Scopolamine (1 mg/kg) impaired spontaneous alternation performance accompanied by an increment in total arm entries. In contrast, SP (0.01-1 micrograms) significantly improved the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance without influencing the scopolamine (1 mg/kg)-induced increase in total arm entries. The effects of SP (0.1 micrograms) on the scopolamine (1 mg/kg)-induced impairment of spontaneous alternation performance were almost completely reversed by pretreatment with WIN 62577 (1 mg/kg), a tachykinin NK-1 receptor antagonist. These results suggest that SP improves the scopolamine-induced impairment of spontaneous alternation performance through the mediation of tachykinin NK-1 receptors.