The effect of ischaemic nerve injury on conduction in the tibial branch of the sciatic nerve has been studied in rabbits. Ischaemic nerve damage wa produced in 40 animals by ligation of the common and external or the internal and external iliac arteries. Conduction in the motor fibres to the small foot muscles was examined at internals after operation, with nerve stimulation in the thigh and at the ankle. Ascending nerve action potentials in the tibial nerve were also studied. In 8 animals there was evidence o widespread degeneration of myelinated fibres; muscle action potentials could not be activated by nerve stimulation nerve action potentials were absent. In contrast there were 14 animals in which arterial ligation appeared to produce little, if any, nerve degeneration; in this group there was no significant change in motor or afferent velocity. The remaining 18 animals showed changes of intermediate severity. While nerve degeneration was extensive, it was never complete, and in 9 nerves examined 5 - 11 days after surgery, conduction block between the thigh and ankle could be demonstrated in surviving fibres. In the other 9 nerves there was no block in surviving fibres but maximal motor velocity was significantly reduced. In no case did the conduction block affect more than a minority of the motor fibres, the remainder undergoing complete degeneration. This was in keeping with the histological appearances which showed a relatively small amount of paranodal and segmental demyelination compared with the amount of Wallerian degeneration.