The relationship between development of intimal thickening and permeability of an arterial autogenous vein grafts in dogs' limbs was examined with horseradish peroxidase used as a tracer. The transendothelial transports increased under conditions of abnormal blood flow, compared with findings with a normal blood flow, and a large amount of horseradish peroxidase infiltrated the subendothelial spaces for 4 weeks after implantation, a time when intimal thickening progressed rapidly. It would thus appear that, under conditions of abnormal blood flow, intimal hyperpermeation would be a contributing factor to the intimal thickening of an autogenous vein graft before an autovein graft had completely adapted to the new environment.