We measured regional blood flow in rat sciatic-posterior tibial nerve using an iodo-[14C]antipyrine autoradiographic technique. Average flow was 12.1 +/- 3.3 ml/100 gm/min, considerably less than that of both gray and white matter structures in the cerebral hemispheres. Perfusion was homogeneous along the proximal-distal extent of this nerve and along its radial axis. Acute occlusion of the femoral artery reduced regional sciatic blood flow. Flow was especially depressed within the endoneurial core of the proximal posterior tibial branch, which lies in a watershed zone between adjacent segmental arterial fields. The topography of the arterial supply to nerves and their abundant collateral circulation may explain the centrifascicular distribution of nerve infarcts, and in part can account for the apparent resistance of peripheral nerves to ischemia.