Rats were undernourished by halving the mother's food intake from the sixth day of pregnancy onwards and through lactation. Subsequently, the young animals were maintained up to 48 days on half the normal diet. The effects of this regime of treatment on myelination in the corpus callosum were investigated by light and electron microscopy. In comparison with controls, the percentage of axons myelinated at 15, 21, and 48 days was reduced in undernourished rats. The number of lamellae constituting the myelin sheath was also reduced at 15 and 21 days, but at 48 days no difference was seen between control and treated rats, suggesting that a catch-up in myelination had occurred. A linear relationship between myelin sheath thickness and axonal diameter was observed in both groups of animals. However, a long-term effect on axonal growth was suggested by findings in the 48-day undernourished animals; in comparison with controls, axonal diameter was reduced, relative to myelin sheath thickness.