β-adrenergic receptors (ARs) are involved in regulating cardiac inotropic and chronotropic responses and peripheral vascular smooth muscle tone. To better understand the roles played by each of the 3 distinctβ-AR subtypes, we used gene targeting to create mice homozygous forβ1 and β2-AR null mutations (-/-). In β1-/- mice, basal heart rate (HR) and blood pressure (BP) were not different from wild type (WT). However, there was no positive chronotropic response to the β-agonist isoproterenol (ISO), which in WT increased HR by 38% (p<0.0001). Echocardiographic measurements of systolic and diastolic LV dimensions and LV fractional shortening (FS) at rest were not different from WT. After ISO,β1-/- mice showed no increase in FS compared with a 71% increase in WT(p<0.005). Both end-systolic (25.7 vs. 7.3 mm, p<0.0001) and end-diastolic (4.2 vs. 3.1 mm, p<0.005) dimensions were larger with ISO inβ1-/- mice. Despite the lack of HR response to ISO, β1-/- mice did increase HR with graded treadmill exercise, but to a lesser degree than WT(4.4% vs. 11.6%). Despite their limited chronotropic and inotropic reserve,β1-/- mice reached similar VO2 and maximal exercise capacity as WT. Inβ2-/- mice, basal HR and BP were also similar to WT. The hypotensive response to ISO seen in WT was attenuated, however it was not totally absent. These findings suggest that, in the mouse, the β1-AR is the subtype predominantly responsible for control of both heart rate and cardiac contractility. Our results also suggest a possible role for the β3-AR in the control of peripheral vascular smooth muscle tone.