Myocardial blood flow in 16 patients before and after coronary bypass surgery in conjunction with coronary angiography, left ventricular function measurement, and graded exercise test was measured. Radioactive 133Xe was injected into the coronary artery or bypass graft and the washout was recorded by an Anger camera. Myocardial blood flow increased in 11 out of 14 patients and decreased in 3 patients. The average flow was 55 (±6) preoperatively and 96(±10) ml 100 g/min postoperatively. Increase of blood flow occurred in both the bypassed left anterior descending coronary artery region and the nonbypassed left circumflex coronary artery region. The postoperative flow increase and the absolute postoperative flow values are higher with saphenous vein than with mammary artery grafts. Statistically significant correlation is not found between myocardial blood flow changes and exercise tolerance. The volume measurements (end diastolic volume, stroke volume, ejection fraction) remained unchanged.