Female Sprague-Dawley rats were made hypertensive by the two kidney/one clip Goldblatt procedure, while control animals were sham-operated. One week later, half of the animals were subjected to a moderate swimming exercise and the other half remained sedentary. Thus, four experimental groups, each consisting of 14 rats, were formed: control animals that were exercised or kept sedentary and corresponding renal hypertensive animals either exercised or sedentary. In hypertensive rats, a significantly increased left ventricular weight and reduced coronary reserve were found. Cardiac hypertrophy in hypertensive rats was characterized by a lower number of capillaries on a tissue cross-section, larger heterogeneity of the capillary net, and a less uniform orientation of capillaries in space. Total length of capillaries in the hypertrophic hearts increased significantly, but less than the increase in cardiac weight, resulting in reduced capillary length density. Chronic swimming for 2 hr/day for a period of 6 weeks, subsequent to a 4-week acclimation period, did not significantly influence any of the investigated indexes of capillaries from hypertrophic hearts. In the normotensive rats, chronic swimming resulted only in a moderate increase in total capillary length associated with a small increase in the left ventricular weight of similar degree. Thus, chronic exercise in normotensive rats induced a moderate increase in total capillary length per left ventricle, while it did not alleviate impaired capillarization of hypertrophic hearts from hypertensive rats. (Clrculation Research 1987;61:641-647)