Abstract.1. ATP, Ca, Mg determinations from arterial and coronary‐venous blood were carried out during routine cardiac catheterization. 2. 10 patients with chronic left heart failure and 10 patients with normal left ventricles were tested. An additional 10 patients with chronic left heart failure were tested only for ATP in arterial and coronary‐venous blood. 3. The blood specimens were taken at rest and during physical exertion as well as before and after the administration of ouabain. 4. In coronary‐venous blood of chronically failing human hearts ATP is increased during physical exertion (arterial = 0.756 μmol ATP/ml whole blood; coronary‐venous = 0.796 μmol ATP/ml whole blood.p<0.005). The results were reproduced in a second group of 10 patients with left heart failure (arterial = 0.654 μmol ATP/ml whole blood; coronary‐venous = 0.693 μmol ATP/ml whole blood. p<0.0005). 5. After the administration of ouabain this effect is no longer detectable in the same patients. 6. The chronically failing human myocardium absorbs essentially more Ca during physical exertion than the normal myocardium does (arterial = 107.4 μg/ml serum; coronary‐vonous = 113.4 μ/ml serum. p<0.0005). 7. In chronic heart failure there was no uptake of Ca after the administration of ouabain. 8. Differences in Mg uptake and release could not be determined in the fa