Internodal length, number of myelin sheath lamellae and axon diameter were estimated in samples of nerve fibres reconstructed from transverse sections (EM and LM) of the L7 ventral spinal root of adult cats, kittens and cat fetuses. The axon calibre spectrum was unimodal in the youngest fetuses, became bimodal at 47 days after mating, i.e. about 2 weeks before birth (63 days after mating) and trimodal at 1 week of postnatal age. Alpha-axons received myelin during the period 40-45 to 50-55 days after mating while gamma-axons became myelinated from 60-63 days after mating to 2-3 weeks of postnatal age. The mean length of the first completely myelinated internodes measured 139 microns and 209 microns in the alpha- and gamma-fibre group respectively. The adult mean values were 1390 microns and 640 microns respectively. The average internodal elongation was x 10 and x 3 in the two fibre groups, whereas the longitudinal growth of the whole ventral root was x 5.4 and x 3.5. A combined linlog function could be fitted to describe the regression between internodal length and axon diameter and was established from birth onwards. The separate alpha- and gamma-fibre samples were best described by linear functions, the alpha internodal length being independent of axon diameter while the gamma internodal length increased linearly with axon diameter. The amount of internodal myelin increased linearly with increasing mantle area of the internodal axon. The mean amount of internodal myelin of alpha-fibres showed two periods of intense growth; one from the start of myelination to 2 months postnatally and the other between 4 and 6 months of age. The mean length of the lamellar spiral forming the myelin sheath showed about the same increase per unit length of axon circumference (about 160:1) in both alpha- and gamma-fibre groups as long as longitudinal internodal growth persisted.