α2-Adrenergic receptors (α2ARs) present in the brainstem decrease blood pressure and are targets for clinically effective antihypertensive drugs. The existence of three α2AR subtypes, the lack of subtype-specific ligands, and the cross-reactivity of α2AR agonists with imidazoline receptors has precluded an understanding of the role of individual α2AR subtypes in the hypotensive response. Gene targeting was used to introduce a point mutation into the α(2a)AR subtype in the mouse genome. The hypotensive response to α2AR agonists was lost in the mutant mice, demonstrating that the α(2a)AR subtype plays a principal role in this response.