Seventy-five patients undergoing coronary artery bypass grafting were randomized to receive injections of papaverine solution or isotonic saline or no injection into the left internal mammary artery (LIMA) used as graft. Blood flow in the LIMA was measured twice-after dissection of the pedicle and before anastomosis to the coronary artery. Blood flow increased significantly in all three groups, but after papaverine injection it was twice as high as in the control groups, increasing by 285% (from 40 +/- 12 to 154 +/- 32 ml/min, p = < 0.0001). The pH of the papaverine solution was only 3, and we advise that surgeons check and correct pH in the papaverine solutions they use, in order to avoid endothelial damage to the LIMA. Based on these results we can recommend papaverine injection into the arterial graft only if the initial flow is low.