Physical fitness of 26 children aged 8-12 years, being 4-7 years after cardiosurgical correction of ventricular septal defect of the heart (VSD) was studied. The results were compared with the ones obtained in 23 healthy children of the same range of age. There were continuously registered in these children the gas exchange values with direct method using progressive exercise test. There were studied minute oxygen uptake, minute carbon dioxide production as well as minute ventilation. In the analysis of physical fitness the following parameters were used: total time of effort, maximal heart rate, maximal power, maximal oxygen pulse, restitution pulse ratio, maximal minute oxygen consumption and ventilatory anaerobi threshold (VAT) determined individually by V-slope method. Comparing both groups of children the statistically significant differences of the VAT values were observed. This evidences the lower physical fitness of children after cardiosurgical correction of VSD.