A comparison was carried out concerning maximal oxygen uptake, oxygen uptake adjustment at the onset of high-intensity exercise, and maximal blood lactate between 10 healthy prepubertal boys and 35 children after repair of cardiac malformations or after Fontan operation. Mean maximal oxygen uptake (VO2) was moderately reduced in children after repair of tetralogy of Fallot or after Mustard or Senning operations and severely reduced after Fontan operations. Conversely, mean half-time of VO2 response was moderately prolonged in children after repair of tetralogy of Fallot or after Senning and Mustard operations and considerably prolonged after Fontan operations. According to our results unfavorable kinetics of VO2 response to physical exercise are present in addition to reduced aerobic power in many of the operated children. Besides being less qualified for endurance performance, these children are also less prepared for short, high-intensity exercise.