1. The conduction velocities of axons supplying the intertransverse caudal muscles of 8-week-old rats were measured. The distribution of conduction velocities was found to be similar to the more commonly studied hind-limb innervation. 2. In animals in which the intertransverse caudal muscles had been tenotomized at birth, however, the conduction velocities attained by both the sensory and the motor nerves by 8 weeks of age were significantly reduced. 3. This effect is limited to growing animals since tenotomy of the intertransverse caudal muscles for the same period in adults had no effect on axonal conduction velocity. 4. Deafferentation of normal intertransverse caudal muscles during development also significantly reduced the conduction velocities attained by the motor innervation, to the same extent as tenotomy had done. 5. These results are discussed in relation to the role of impulse traffic in the development of neuronal dimensions.