A basic study of the horseradish peroxidase (HRP) method was done using the sciatic nerves of rats. The nerve was crushed with a needle and HRP was injected around the nerve at the site 7-8 cm from the spinal cord. The nerve at the injection site showed an intense positive reaction to HRP 3 to 48 hours after the injection. When the nerve was ligated between the spinal cord and the injection site, HRP accumulated at the distal part of the lesion after 24 hours, and was not detected at the proxismal part of the lesion. When unilateral sciatic nerve was ligated and HRP injected bilateral nerves, motor neurons of the ligated side only did not show positive reaction to HRP. These results showed that HRP was transported by retrograde axonal flow in the nerve. Motor neurons of the cord began to show intense positive reaction 30 hours to 14 days after the injection of HRP around the nerve. The rate of retrograde axonal flow in the nerve was calculated to be 2.6 mm/hr (62.4 mm/day). Neurons in the spinal ganglia were stained intensely 24 hours after the injection of HRP, which suggests more rapid axonal flow in sensory neurons that in motor neurons.