The axon diameter (d) and the number of myelin sheath lamellae (nl) were estimated in electron micrographs of cross sectioned ventral spinal roots of the seventh lumbar nerve of adult cats, kittens and cat fetuses. Myelination started between the 40th and the 45th day after mating (about 3 weeks before birth). From birth onwards the calibre spectrum consisted of a group of small fibres and a group of large fibres. During the first two postnatal months the point distribution of the number of myelin lamellae plotted against the axon diameter appeared more or less linear. This point distribution pattern became curved from the 10th postnatal week onwards, and a function of the general expression: nl = C0 + C1d + C2 log(d) could be fitted to describe the regression. In adult cats, the mean number of myelin lamellae/axon diameter ratio was about 30 in the group of small and about 15 in the group of large fibres. Observations on immature animals indicated that the difference between the two fibre groups could have been established already during the third postnatal week. Considerable myelin production took place after the age of 1 year. The cross sectional area of the myelin sheath in the alpha fibre group increased about 10% after the age of 1 year, while in the gamma fibre group it increased as much as about 85%. The absolute amount of myelin per unit fibres. Differences and similarities between myelin production in the two fibre groups are discussed.