1. Forty-six determinations of the oxygen unsaturation, i.e., the difference between the venous oxygen and the total oxygen capacity of the hemoglobin, have been done in five patients with compensated circulatory disturbances.
2. Values within normal limits and near or below the normal average were found in four patients with regular pulse.
3. In one patient with mitral insufficiency and arrhythmia perpetua extremely varying values were encountered. Six determinations were made; in one instance the value was within normal limits, in five above, and in one of those even higher than in most cases of incompensated heart lesions.
4. A comparison is drawn between the directly determined oxygen unsaturation in these patients and the oxygen consumption calculated from previous experiments by the writer, where the blood flow was determined directly by the nitrous oxide method. A close parallelism is found.