Napamezole is a selective α2-adrenergic receptor antagonist, evidenced by its reversal of clonidine-induced effects in the rat vas deferens. It also exhibits α1-antagonistic activity, antagonizing methoxamine-induced contractions with a Kb of 135 nM. This compound is a useful pharmacological tool for studying adrenergic receptor function in both in vitro and in vivo neuropharmacological research.