During coronary artery bypass surgery graft flows were measured. The measurements were made after cardiopulmonary bypass had been discontinued. The recordings were made over a ten year period and a total of 1,151 grafts in 720 patients are included. Except where the flow down a graft was large, 20 mg of Papaverine were injected into the graft and this usually resulted in the flow increasing by at least 100%. A satisfactory response was not achieved in 32 of the 538 grafts in which Papaverine was injected but subsequent probing of the distal anastomosis achieved a markedly improved flow in 80%.