The capillary proliferative activity in heart and skeletal muscle was studied by autoradiography after in vivo injections of [3H]thymidine in swimming exercised rats. Swimming exercise led to hypertrophy of the myocardium and the muscle fibers of the fore- and hindlimbs. When compared with normal controls a highly significant increase in nuclear incorporation of [3H]thymidine was found in the cells of the myocardial capillary walls. No increased incorporation of [3H]thymidine was found in capillary wall cells of fore- and hindlimb muscles. The findings confirm previous observations of a significant neoformation of myocardial capillary blood vessels in swimming-induced cardiac hypertrophy, whereas any capillary neoformation in the hypertrophying skeletal muscles of this experimental model is insignificant.