During reconstructive arterial surgery on nine patients suffering from obliterating femoral arteriosclerosis, the number of platelets, the percentage of reversible platelet aggregates, and the plasma concentration of a platelet release marker (platelet factor 4, PF-4) were determined. Blood samples were collected simultaneously from the popliteal vein of the leg operated upon, and from the superior caval vein and radial artery at various stages of the operative procedure. Only slight changes in platelet number were observed. Platelet aggregates increased in blood from all sampling sites during ischaemia, but corresponding values were in general lower in arterial than in venous blood. The percentage of reversible platelet aggregates in the popliteal vein rose considerably during ischaemia, and the rise correlated well with the arterial occlusion time (Rs = 0.96). PF-4 concentration rose throughout the procedure. Although the clinical implications of these findings are not clear, they seem to underline the importance of a swift but careful surgical technique and a short period of main vessel occlusion.