Coronary vein graft flow and aortic or intragraft pressure were recorded from 31 grafts in 21 patients under stable hemodynamic conditions before and after the administration of papaverine hydrochloride (10 mg) into the graft. Resistance to flow was calculated and expressed in peripheral resistance units (pru). Flow was greater in left-side grafts, 73 ± 23 ml/min, than on the right, 53 ± 25 ml/min. Respective resistance figures are 1.2 ± 0.4 on the left and 2.6 ±1.6 on the right. Following administration of papaverine the average flow increased on the left by 93%, and on the right by 110%; corresponding reduction in resistance occurred on either side, but with less difference between left and right. Three of four grafts known to have occluded in this series had flow less than 30 ml/min and high resistance values between 2.5 and 8.0 pru.