Myocardial blood flow has been studied in 7 trained and 7 untrained healthy volunteers by means of the argon method. Measurements were carried out at rest, during stepwise increase in arterial hypoxia down to 8.7 vol % and during heavy exercise comparable to 65% of delta VO2 max. Measurements were carried out during hemodynamic and respiratory steady state conditions. Hemodynamic parameters were measured or even monitored. It could be shown that myocardial blood flow, myocardial oxygen consumption and myocardial oxygen consumption per heart beat are less in the trained state compared to the untrained one. The differences are particularly pronounced during heavy exercise. The reaction of the coronary conductance vessels to acute arterial hypoxia is not influenced by training effects. The surplus of oxygen and substrates supplied to the heart by the coronary circulation is well maintained during exercise and hypoxia.