21 clinically asymptomatic children aged 5.5 to 15.5 years were examined by bicycle exercise 1 to 9 years after surgical repair for tetralogy of Fallot. The results were compared to normal values gained from 169 healthy children (10 age groups of girls and boys). Submaximal exercise testing was performed in three stages of 1, 2 and 3 Watts per kg body weight, followed by 2 minutes of recovery. The following results were gained: 7 of the 21 patients showed rhythm disturbances during the exercise testing. Only three of them had documented arrhythmias before this date. Another 3 children with normal exercise test had arrhythmias in their former history. Thus total incidence of documented arrhythmias is 48 percent in contrast to only 1.2 percent in the control group. Using this weight related exercise test the heart rates of the patients were significantly lower during exercise than those of the control group. The differences are most evident during the maximal exercise period. The ability of LVET-shortening with increasing heart rates was significantly reduced in the patients. PEP was significantly prolonged. Both phenomena lead to a relatively shorter diastole than in normal subjects. This must result in a hemodynamical disturbance under the conditions of high frequencies. Blood pressure monitoring is not very valuable for the interpretation of hemodynamic changes. There are signs of alterations of hemodynamics as well as of the heart rhythm in the patients after surgical repair for the tetralogy of Fallot, even if the patients feel very well. By short time submaximal exercise testing it is possible to detect these alterations.