There is mechanistic rationale to suggest differential effects of angiotensin-converting enzyme (ACE) inhibition and angiotensin II type 1 (AT 1)-receptor antagonism on ventricular remodeling after myocardial infarction (MI). We compared the effects of ACE inhibition, AT 1-receptor antagonism, and their combination on post-MI ventricular remodeling in rats. We induced MI in 62 rats, which then received one of four treatments:(a) placebo;(b) the ACE inhibitor, enalapril;(c) the AT 1-receptor antagonist, losartan; and (d) enalapril and losartan in combination. Two weeks after MI, we examined:(a) heart weight (HW)/body weight (BW) ratio;(b) nonmyocyte cellular proliferation in the noninfarct zone by using proliferating cell nuclear antigen staining; and (c) collagen content within the noninfarct zone. Placebo-treated, infarcted rats developed significant increases in HW/BW ratio (p< 0.001), left ventricular (LV) volume (p< 0.01), nonmyocyte cellular proliferation (p< 0.04), and collagen content (p< 0.01) compared with noninfarcted controls. Enalapril, losartan, and combination therapy limited the increase in HW/BW ratio (all p values< 0.01 vs. placebo). Enalapril inhibited nonmyocyte proliferation (p< 0.01 vs. placebo), whereas losartan had a smaller effect (p= NS vs. placebo; p< 0.03 vs. enalapril); combined treatment also reduced nonmyocyte cellular proliferation but did not reach statistical significance (p= 0.08 vs. placebo). Enalapril and combination treatment significantly diminished collagen content (both p values< 0.01 vs. placebo), whereas losartan did not. Thus, ACE inhibition and AT 1-receptor antagonism equally limited myocardial hypertrophy after MI in rats, but ACE inhibition more effectively prevented nonmyocyte cellular proliferation and collagen deposition in the noninfarcted myocardium. Combination therapy was no more effective than was ACE inhibition alone. These data suggest that the myocyte hypertrophic response after MI is strongly influenced by activation of the AT 1 receptor, whereas nonmyocyte cellular proliferation and collagen deposition result, in part, from mechanisms separate from AT 1-receptor activation.