In the study of electrocardigrams(ECG's) and exercise performance of competitive swimmers, we compared the resting electrocardiographic findings, maximal exercise time and heart rate determined by treadmill exercise test between swimmers and controls. in addition, the effects of an intensive training of 7~9 weeks duration on these parameters were also studied in the swimmers. The study groups consisted of 22 female competitive swimmers with mean age and career of 15.2 and 5.7 years, respectively and 22 matched controls. In the swimmers, the incidence of sinus bradycardial was higher and mean heart rate was lower than those in controls. In a few case of the swimmers, ECG's before the intensive training revealed the first degree heart block, ectopic atrial rhythm and high voltage, but there was no significant changes in the incidences of these findings after the training. In the swimmers before training, maximal exercise time was longer and maximal heart rate was lower than those in the controls, as expected, and the intensive training induced slight but significant changes of the same directions in these parameters compared with those before the training. These facts suggest that in the well-trained competitive swimmers, a short period of intensive training induces no significant changes in ECG's at rest but may prolong exercise time and decrease maximal heart rate.