The H reflex evoked in the plantar muscles of the rat hind limb was studied. The latency of the reflex and the conduction velocity of the sensory and motor nerves in the reflex arc were determined in rats of different sizes. Sensory conduction velocities were 38.8 ± (sd)5.8 m/s in 100- to 150-g rats and 60 ± 5.9 m/s in 450- to 500-g rats compared with motor conduction velocities of 30.1 ± 2.8 m/s in 100- to 150-g and 55.0 ± 6.9 m/s in 450- to 500-g rats. Both motor and sensory velocities were directly related to rat size. However, the latency of the H reflex evoked at the ankle was relatively independent of rat size. It was 6.62 ± 0.36 m/s in 100- to 150-g rats and 6.05 ± 0.43 m/s in 450- to 500-g rats with a mean for 39 rats from 100 to 500 g of 6.44 ± 0.41 ms. This method of determining motor and sensory conduction velocities in the rat is simple, does not necessitate killing the animal, and should prove useful in the study of experimental neuropathies.