Gradual pressure overload was induced by abdominal aortic constriction in male rats on postnatal d 6 (AC6) or 2 (AC2). At the age of 8 wk, the systemic blood pressure was measured, and the contractile performance of the left ventricle (LV) was assessed after acute ligation of the ascending aorta in open chest anesthetized animals. The LV free wall was used for the determination of collagen concentration and morphometric analysis of cardiac myocytes and capillaries. Aortic constriction resulted in LV hypertrophy, which was more pronounced in AC2 (by 71%) as compared with AC6 (by 34%) groups and correlated closely with the degree of pressure overload (r = 0.88 and 0.80, respectively). The right ventricular weight was increased by 13% in the AC2 group only. Contractile performance of the LV of aortic constricted rats was significantly higher before as well as after the acute load, but the average functional reserve was unchanged in both experimental groups. Although the maximum value of the rate of pressure development increased linearly with the degree of ventricular hypertrophy in the AC6 group(r = 0.82), a negative correlation was observed in the AC2 animals(r = -0.61). The density of myocytes was decreased, and the calculated average myocyte cross-sectional area was increased in aortic constricted rats, but the coronary capillary density and myocardial concentration of collagen remained constant. Thus, in spite of the larger cardiac growth response in the younger age group, the capillary proliferation and collagen formation were proportional to the ventricular hypertrophy. Therefore, the degrees of overload and hypertrophy do not seem to be limiting factors. Pressure overload induced in newborn rats can be a useful model for the study of mechanisms that control either the growth and differentiation of myocardium soon after birth, as well as the transition from compensated to decompensated hypertrophy at later stages.