Using Implanted pulsed Doppler microprobes sutured on saphenous bypass grafts in ten patients we studied, 6 h after cardiac surgery, the effects of 5 and 10 μg. kg−1. min−1of dobutamine on mean (&OV0422;m, systolic (&OV0422;d), and diastolic (&OV0422;d) coronary bypass graft flows, as well as on coronary systolic (&OV0422;dand diastolic (∞&OV0422;d) blood volumes entering the myocardium per cardiac beat. &OV0422;mincreased during the inotropic stimulation from 61.8 ± 19.2 to 81.1 ± 21.8 ml.min−1(P< 0.001) and resulted from an unchanged &OV0422;mand from a large increase in &OV0422;d(P< 0.01). &OV0422;dincreased more than did diastolic arterial pressure and was related to rate pressure product taken as an index of myocardial oxygen consumption (r = 0.76,P< 0.001). Despite the dobutamine-induced increase in heart rate (P< 0.01), &OV0422;d, and &OV0422;dthe systolic and diastolic inflow volumes per cardiac beat were unchanged. We conclude that increased myocardial blood supply through the saphenous vein bypass graft during inotropic stimulation by dobutamine resulted from different systolic and diastolic events. The oxygenated Mood volume entering the coronary vascular bed per beat was unchanged despite tachycardia.