By the use of the xenon-residue detection technic, myocardial blood flow (MBF) was assessed 3-17 months after surgery in nine patients with an aorta-to-left anterior descending bypass (LAD-BP), in nine patients with an aorta-to-right coronary artery bypass (RCA-BP), and in two patients with both LAD-and RCA-BP. In addition, MBF was registered in the corresponding main coronary artery. In each case coronary angiography revealed a patent vein graft. LAD-BP flow averaged 58 ml/min/100 g, this being slightly lower than the average flow value recorded after injection into the main left coronary artery of these patients (64 ml/min/100 g). LAD-BP flow did not differ from the value recorded in 20 normal subjects (63.9 ml/min/100 g). In contrast, average RCA-BP flow amounted to 47.1 ml/min/100 g and was significantly higher than the values recorded in the obstructed main RCA of these patients (22.1 ml/min/100 g) (P< 0.0025) or those observed in 20 patients with severe disease of the right coronary artery (34.9 ml/min/100 g) (P< 0.025); no difference was seen between RCA-BP flow and the average of 20 normal subjects (46.1 ml/min/100 g). In six patients a two-to threefold increase of BP flow was found during exercise (50-100 w). Total coronary vascular resistance showed a similar behavior to MBF, not differing in the bypass area from the values obtained in the normal control groups. It is concluded that the venous bypass procedure substantially increases perfusion at rest and during exercise and that a good correlation exists between MBF and postoperative angiography.