A comparative study on electrocardiograms (ECG) at rest was done between 72 baseball players with a mean career of 11.9 years and 20 normal controls. In addition, ECG of 36 players and all controls during treadmill exercise test by Bruce protocol, carried out until approximately 90% of the predicted maximum heart rate, were reviewed and also echocardiograms (UCG) of four players were analyzed. In ECG at rest, PR interval prolongation and high voltage of QRS complex were noted only in the players, two (2.8%) and nine (12.5%) of them, respectively. The incidence of early repolarization was significantly higher in the controls. Bruce tests were positive in nine players (25%) and in one control (5%), and the exercise times on treadmill were significantly longer in the players than in the conrols (13.9 vs 12.5 min). In eight of the nine Bruce-positives, abnormal ST-segment depression of horizontal type was seen only in inferior leads, and in two it disappeared during the test before target heart rate was achieved. In Bruce-positive players, the incidences of sinus bradycardia and high voltage were significantly longer than that in the negatives as well as in the controls. Analyses of ECG in four players including one Bruce-positive case revealed left ventricular hypertrophy or dilatation along with high voltage in three, and right venricular dilatation in one. These facts suggest that, although adaptive changes in the heart of baseball players are reflected on ECG by sinus bradycardia, high voltage and prolongation of QRS duration and on ECG by ventricular hypertrophy or dilatation, these changes have little to do with the results of exercise test.