In humans, the effect of angiotensin converting enzyme (ACE) gene polymorphisms in cardiovascular disease is still controversial. In the rat, a microsatellite marker in the ACE gene allows differentiation of the ACE gene polymorphism among strains with different ACE levels. We tested the hypothesis that this ACE gene polymorphism determines the extent of cardiac fibrosis induced by isoproterenol (ISO) in the rat. We used a male F₂ generation (homozygous LL and BB ACE genotypes determined by PCR) derived from two rat strains (Brown-Norway and Lewis) that differ with respect to their plasma ACE activities. For induction of left ventricular hypertrophy (LVH) and cardiac fibrosis, rats were infused with ISO (5 mg per Kg per day) or saline (control) for 10 days and sacrificed at day 1 post last-injection. The interstitial collagen volumetric fraction (ICVF), collagen I and fibronectin content, but not collagen III content, were significantly higher in the homozygous BB rats than in homozygous LL rats. Differences in metalloprotease-9 (MMP-9), but not in MMP-2 activities as well as in cardiac cell proliferation were also detected between LL and BB rats treated with ISO. LV ACE activity was higher in BB rats than LL rats and correlated with ICVF (r =0.61, P<0.002). No changes were observed in plasma ACE activities, angiotensin II (Ang II) plasma or LV levels, plasma renin activity, and ACE and Ang II type 1 receptor (AT1R) mRNA levels in the LV of rats with the two different ACE polymorphisms. ISO induced a similar degree of LVH (assessed by an increase in LV weight 100/body weight, LV to right ventricle (RV) ratio and LV protein content) in LL and BB rats. We concluded that rats in the F₂ generation with high plasma ACE activity developed more fibrosis but a similar degree of LVH as compared to rats with low plasma ACE activity.