Central venous oxygen saturation (CVSO2) was measured in 31 patients with myocardial infarction. CVSO2 correlated well with the patients' clinical course. In those patients not in heart failure, mean + SEM for CVSO2 was 70 + 1%. When heart failure was present, CVSO2 averaged 56 + 1%. When both heart failure and shock were present, CVSO2 averaged 43 + 1%. In nine patients, serial determinations of arterial oxygen saturation and CVSO2 were made. In 22 of 26 instances, either a fall in CVSO2, was accompanied by an increase in the arteriovenous oxygen saturation difference or an increase in CVSO2 was accompanied by a decrease in arteriovenous oxygen saturation difference. Serial measurements of CVSO2 appear to be a useful method of monitoring changes in myocardial function in patients with myocardial infarction.Additional Indexing Words: Mixed venous oxygen saturation Central venous pressure MEASUREMENT of cardiac output and intracardiac pressures is used to assess myocardial function in the catheterization laboratory.