In 19 median and 10 ulnar nerves of 28 healthy young volunteers, the maximum orthodromic sensory conduction velocity was studied along one or two fingers (the third and/or the fifth), the palm, and the forearm. In five 20 to 32-yr-old males and in a newborn male, post-mortem samples of either the median or the ulnar nerve were obtained from the finger (either the third or the fifth), the palm, the wrist and the forearm in order to study the size distribution of external fibre and axon diameters, and the fibre internodal length. In 2374 infants, children and adolescents, the height, length of both the third and the fifth finger, length of the palm, and length of the forearm were measured. The results showed (i) a significantly slower conduction velocity along the fingers; (ii) a significantly shorter internodal length without remarkable fibre diameter changes in the same nerve segments; (iii) a significantly smaller elongation of these body parts; (iv) a significant correlation between these data. In conclusion, internodal length seems to play an important role in governing conduction velocity of myelinated peripheral nerve fibres.