Blood flow and pressures around a canine femoral arteriovenous fistula, constructed from an autogenous carotid artery, were measured to determine the pressure-flow relationships within the fistula, as well as to determine the presence or absence of retrograde flow in the distal artery and vein. The findings indicate that retrograde arterial flow occurs only when the proximal femoral artery is occluded and, even then, the contribution to fistula flow is small and well below control femoral arterial flow. Retrograde distal venous flow is negligible. Pressure within the fistula is high at the arterial end but is rapidly dissipated across the loop fistula to equal systemic venous pressure on the venous side. Modest distal venous hypertension accompanies this type of fistula. For these reasons, a loop-type, arteriovenous fistula is preferable to other types of fistulas for most clinical surgical indications.