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. Radioactive 133-Xe 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 three patients. The average flow was 55 (plus or minus 6) preoperatively and 96 (plus or minus 10) ml 100 g/min postoperatively. Increase of blood flow occurred in both the bypassed left anterior descending coronary artery (LAD) region and the nonbypassed left circumflex coronary artery (LCIR) 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 exerice tolerance. The volume measurements (end diastolic volume, stroke volume, ejection fraction) remained unchanged.