A ngiotensin II (Ang II), a potent hypertrophic stimulus, causes significant increases in TGFb1 gene expression. However, it is not known whether there is a causal relationship between increased levels of TGF-β1 and cardiac hypertrophy. Echocardiographic analysis revealed that TGF-β1–deficient mice subjected to chronic subpressor doses of Ang II had no significant change in left ventricular (LV) mass and percent fractional shortening during Ang IItreatment. In contrast, Ang II–treated wild-type mice showed a >20% increase in LV mass and impaired cardiac function. Cardiomyocyte cross-sectional area was also markedly increased in Ang II–treated wild-type mice but unchanged in Ang II–treated TGF-β1–deficient mice. No significant levels of fibrosis, mitotic growth, or cytokine infiltration were detected in Ang II–treated mice. Atrial natriuretic factor expression was ∼6-fold elevated in Ang II–treated wild-type, but not TGF-β1–deficient mice. However, the α- to β-myosin heavy chain switch did not occur in Ang II–treated mice, indicating that isoform switching is not obligatorily coupled with hypertrophy or TGF-β1. The Ang IIeffect on hypertrophy was shown not to result from stimulation of the endogenous renin-angiotensis system. These results indicate that TGF-β1 is an important mediator of the hypertrophic growth response of the heart to Ang II.