The properties of spinal motoneurones of normal and dystrophic mice (129/ReJ) were examined with intracellular electrodes. 2. The following parameters of spinal motoneurones showed no significant differences between normal and dystrophic mice: resting and action potentials, the amplitude and duration of after-hyperpolarization, rheobasic current for excitation, threshold for excitation of the somadendritic membrane (IS-SD inflexion) and input resistance. 3. The changes in motoneurone properties observed 13-16 days after section of the sciatic nerve (axotomy) were similar in both normal and dystrophic mice. 4. The axonal conduction velocity of motoneurones in dystrophic mice was about ten times slower than that in normal mice. The conduction velocity of the sciatic nerve was only about 25% slower in dystrophic mice than in the normal animal. The estimated ventral root conduction velocity as well as the observed dorsal root conduction velocity in dystrophic mice was at least twenty times slower than that in normal mice. 5. In dystrophic mice, spinal motoneurones often showed multiple discharges in response to single, antidromic stimuli. The site of initiation of multiple discharge was located in the motor axon rather than in the motoneurone cell body. 6. In dystrophic mice, nerve impulses were transmitted from fibre to fibre ('cross-talk'). The site of impulse transmission among nerve fibres was near the distal portion of the spinal roots. 7. Synaptic potentials and peripheral reflex discharges evoked by stimulation of the dorsal roots showed a longer latency and were more prolonged in dystrophic mice than in the control mice. 8. The motoneurone properties of dystrophic mice showed no tendency of progressive changes with age ranging from 63 to 148 days. 9. It is concluded that the properties of motoneurone cell bodies examined in dystrophic mice are indistinguishable from those in normal mice and that the only abnormality in motoneurones of the former residues in the motor axon. 10. It is suggested that integrity of the discharge pattern of spinal motoneurones in dystrophic mice is interfered by anomalous impluse transmission in the motor axons and that the motoneurones in dystrophic mice are a homogeneous group rather than a mixture of "normal" and "abnormal" neurones.
