We studied nine freshmen and 14 senior oarsmen undergraduates during seven months of training and compared them with 17 age and sex-matched sedentary control subjects in order to assess the influence of heavy physical exercise on cardiac dimensions and maximal oxygen uptake. Standard M-mode echocardiographic techniques were used. At the start of the season senior oarsmen had a greater left ventricular end-diastolic dimension, and a thicker interventricular septum and posterior left ventricular wall than control subjects and freshmen oarsmen. The two latter groups did not differ from each other. During the training period there was a slight and gradual increase in left ventricular end-diastolic dimension, and interventricular septum and posterior wall thickness in freshmen. In seniors only left ventricular end-diastolic dimension increased significantly. Maximal oxygen uptake showed a distinct increase between the fourth and seventh month during the period of intensive rowing training. There was no relation between echocardiographic variables and maximal oxygen uptake. A combination of heavy dynamic and static exercise can thus lead to significant changes in both left ventricular wall thickness and chamber size within months. Echocardiographic variables measured at rest cannot be used as a suitable index of performance capacity.