1. Maximum motor nerve conduction velocities were determined under standard conditions for the median and ulnar nerves in the right forearm in males: (a) from the general population, (b) from a subnormality hospital, and (c) in males with the XYY sex chromosome abnormality. Distal latencies for these nerves were also measured.
2. No significant differences in maximum peripheral motor nerve conduction were found between the two control populations from the general population and from the subnormality hospital.
3. The maximum motor conduction velocity in the median nerve in the XYY group was significantly slowed when compared with both control groups. In the case of the ulnar nerve, maximum motor conduction was significantly slowed when compared with the general population, but not when compared with the subnormality hospital group.
4. There were no significant differences in the mean distal latencies of either the median or ulnar nerves between the two control groups.
5. In the XYY group, the distal latency in the ulnar nerve was significantly prolonged when compared with both control groups. In the case of the median nerve, the distal latency was prolonged significantly when compared with the general population; when the comparison was made with the subnormality hospital group significant prolongation was also observed but was less marked.