α1-adrenergic receptors (ARs) play a major role in blood pressure regulation. The three α1-AR subtypes (A/C, B, and D) stimulate contraction of isolated arteries, but it is uncertain how different subtypes contribute to blood pressure regulation in the intact animal. We studied the role of the α1A/C subtype by using gene knockout. α1A/C knockout (KO) mice were viable and overtly normal. The LacZ reporter gene replaced α1A/C coding sequence in the KO, and β-galactosidase staining was present in resistance arteries and arterioles, but not in the thoracic aorta or its main branches. By tail cuff manometer and arterial catheter in conscious mice, α1A/C KO mice were hypotensive at rest, with an 8-12% reduction of blood pressure dependent on α1A/C gene copy number. A61603, an α1A/C-selective agonist, caused a pressor response that was lost in the KO and reduced but significant in heterozygous mice with a single copy of the α1A/C. A subtype-nonselective agonist [phenylephrine (PE)] caused a pressor response in KO mice, but the final arterial pressure was only 85% of wild type. The baroreflex was reset in the KO, and heart rate variability was decreased. After baroreflex blockade with atropine, PE increased blood pressure but did not change heart rate. Cardiac and vascular responses to the β-AR agonist isoproterenol were unchanged, and the arterial lumen area was not altered. We conclude that the α1A/C-AR subtype is a vasopressor expressed in resistance arteries and is required for normal arterial blood pressure regulation. α1A/C-selective antagonists might be desirable antihypertensive agents.