T o investigate the physiological role of the α_1D-adrenergic receptor (α_1D-AR) subtype, we created mice lacking the α_1D-AR (α_1D^–/–) by gene targeting and characterized their cardiovascular function. In α_1D^–/– mice, the RT-PCR did not detect any transcript of the α_1D-AR in any tissue examined, and there was no apparent upregulation of other α₁-AR subtypes. Radioligand binding studies showed that α₁-AR binding capacity in the aorta was lost, while that in the heart was unaltered in α_1D^–/– mice. Non-anesthetized α_1D^–/– mice maintained significantly lower basal systolic and mean arterial blood pressure conditions, relative to wild-type mice, and they showed no significant change in heart rate or in cardiac function, as assessed by echocardiogram. Besides hypotension, the pressor responses to phenylephrine and norepinephrine were decreased by 30–40% in α_1D^–/– mice. Furthermore, the contractile response of the aorta and the pressor response of isolated perfused mesenteric arterial beds to α₁-AR stimulation were markedly reduced in α_1D^–/– mice. We conclude that the α_1D-AR participates directly in sympathetic regulation of systemic blood pressure by vasoconstriction.