The effects of grafting segments of femoral vein in the dog have been studied. The venous segments, either reversed or left in situ, were interposed in the course of the femoral artery; these 'arterialized grafts' were compared with a segment of vein grafted into the venous stream, with normal femoral artery and vein. Histological changes and alterations in fibrinolytic activity were studied after 15, 30 and 90 days. The two types of arterialized venous graft showed similar aspects: normal endothelium (scanning electron microscopy and F VIII immunofluorescence), marked thickening of the smooth muscle cell layer, disappearance of intimal fibrinolytic activity (Todd's technique, quantified by morphometric analysis), associated with a marked increase of antiplasmin activity (Noordhoek Hegt's technique), increase in the activity of vascular plasminogen activator (measured with chromogenic substrates). The venous grafts in the venous stream showed no changes compared to the normal femoral vein. The proliferation of smooth muscle cells following exposure of a segment of vein to arterial conditions appears to be associated with increased synthesis of both an inhibitor of fibrinolysis (antiplasmin), and of vascular plasminogen activator. The contribution of these two factors to the development of the morphological changes in the vessel requires further study.