Male rats undernourished from the 18th day of gestation till 100 days of age were nutritionally rehabilitated until 200 days of age. Six control and six experimental rats at each of 25, 50, 100, and 200 days of age were killed by perfusion with buffered 2.5% glutaraldehyde. Pieces of optic nerve from just behind the left eye of each rat were postfixed in osmium tetroxide and embedded in resin. Estimates of the total number of fibres and their mean diameter and size-frequency distributions were made for each nerve. Rats undernourished until 25 days of age had a mean +/- SE of 57,464 +/- 6,778 fibres per optic nerve. This represented a 36% deficit (P less than 0.01) compared with the 25-day-old control value of 89,778 +/- 6,625. In addition, these undernourished rats had proportionately more small fibres than the age-matched controls. This resulted in a significant deficit in mean fibre diameter at this age. These deficits and distortions disappeared in all the older rats studied, despite the continued undernourishment of some animals up to 100 days of age. It appears that even the lengthy period of undernutrition imposed in the present experiments could not produce a permanent deficit in optic nerve fibre number, size, and size distribution. It is suggested that the deficits seen initially at 25 days of age may have been due to a temporary delay in the growth and development of these nerve fibres.