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KCNH3 (BEC1) is a member of the ether-à-go-go (KCNH) family of voltage-gated K+ channels. The aim of this study was to determine the pharmacological profiles in vitro and in vivo of a KCNH3 inhibitor N-(4-fluorophenyl)-N′-phenyl-N′′-(pyrimidin-2-ylmethyl)-1,3,5-triazine-2,4,6-triamine (ASP2905). We analyzed the effects of ASP2905 on channel activity in vitro and its neuropharmacological properties in young and aged rats as well as in mice. ASP2905 potently inhibited potassium currents in CHO cells expressing KCNH3 (IC50 = 9.0 nM). In contrast, ASP2905 (≤ 10 μM) minimally bound with low affinities to 55 transmembrane proteins. ASP2905 (0.1 μM, 1 μM) decreased the frequency of spontaneous inhibitory postsynaptic currents in cultured rat hippocampal neurons. In mice, ASP2905 reversed the disruption of spontaneous alternation behavior induced by MK-801 and scopolamine (minimum effective dose of ASP2905: 0.0625 mg/kg, po). ASP2905 ameliorated the cognitive deficits of aged rats in step-through passive avoidance (0.0313 and 0.0625 mg/kg, po) and Morris water-maze tasks (0.01 mg/kg, po) and effectively penetrated the brain. The mean plasma and brain concentrations of ASP2905 reached their maxima (Cmax = 0.399 ng/ml and 1.77 ng/g, respectively) 1 h after a single oral administration and then decreased (t1/2 = 1.5–1.6 h) (brain plasma ratio = 2.7–4.9). The present study suggests that ASP2905 is a selective, orally administered inhibitor of KCNH3, which can enhance cognitive performance.