Fifteen patients with intractable angina pectoris underwent coronary angiography and coronary arterial bypass graft surgery. After the operation, a continuous infusion of krypton-81m was delivered into each graft. A gamma camera and multichannel analyzer were used to record the regional myocardial distribution of perfusion provided by each graft. The disappearance of myocardial activity at the end of each infusion was used to calculate the flow per unit volume in the myocardial distribution provided by each vessel. Myocardial perfusion provided by the grafts to all the major coronary arteries were recorded individually as high spatial resolution images. Myocardial flow rates in the distribution of each graft were measured using the washout of krypton-81m. The rates were 0.5 to 2.0 ml/g per min in the 13 patients with no evidence of previous myocardial infarction. Krypton-81m was infused into grafts to the left anterior descending coronary artery in two patients with a history and electrocardiographic evidence of previous anterior myocardial infarction. The grafts provided poor perfusion to the anterior and apical portions of the ventricles. An experimental model of myocardial perfusion was used to demonstrate that the washout of krypton-81m can be used to measure flow per unit volume within or above the physiologic range.