The hemodynamic and myocardial effects of chronic treatment (5 months) with converting enzyme inhibitor (CEI, enalapril, group T1, n = 10) or a beta 1 blocker (atenolol, group T2, n = 13) of normotensive mature male Wistar rats were studied and compared to untreated younger rats (group C0, 11 months old, n = 14) and to untreated age-matched controls (group C1, 16 months old, n = 14). Heart rate was significantly decreased in T2 rats but unchanged in the T1 group. There was a significant decrease in systolic and diastolic arterial blood pressure (BP) in both treated groups (systolic T1:90 +/- 17, T2: 87 +/- 10, vs. C1: 102 +/- 11 mm Hg, and diastolic T1: 60 +/- 17, T2: 59 +/- 9, vs. C1: 73 +/- 13 mm Hg). Cardiac output was unchanged by therapy but stroke volume was significantly increased in the T2 group (0.24 +/- 0.03 vs. 0.19 +/- 0.04 cm3 in C1). Total peripheral resistance was significantly decreased in T1 and T2 groups. Left ventricular weight to body weight ratio (LVW/BW) was significantly decreased in T1 (1.74 +/- 0.11 vs. 2.02 +/- 0.22 in C1) but unchanged in T2 (1.90 +/- 0.15 vs. 2.02 +/- 0.22). The left ventricular passive pressure-volume curve was significantly rightward shifted by beta-blocker therapy (T2) but unchanged by CEI (T1). This corresponds to a decrease of chamber stiffness k in the T2 group compared to C1 and T1 groups. A significant fibrosis was seen in group T2 (mean collagen fiber thickness of 3.03 +/- 0.24 vs. 2.8 +/- 0.15 micron) but not in the T1 group (2.94 +/- 0.14 vs. 2.8 +/- 0.15 micron). Thus, despite the similar decrease in BP induced by the CEI and the beta blocker, the cardiac effects of the two drugs were opposite: CEI decreased LVW/BW with no change in the left ventricular pressure-volume curve and myocardial fibrosis whereas beta blocker did not change the LVW/BW, but induced a left ventricular dilation associated with a decrease in chamber stiffness and an increase in subendocardial fibrosis.