The guinea pig, like the human, initiates the period of rapid brain growth in utero and thus provides a model for measuring the effects of maternal malnutrition on intra-uterine brain growth. In these studies the newborn of undernourished guinea pig mothers showed significant reductions in body weight and brain weight, cellularity, protein, cholesterol, cerebroside, and sulfatide contents. The reductions in wet brain weight and protein content were significant for cerebellum but not for cerebrum. Animals undernourished in utero and fed normally after birth showed normal whole brain weight, cerebroside and sulfatide contents, and normal cerebrum cellularity by adulthood. However, the type of cells increasing in the cerebrum during postnatal rehabilitation is unknown. Wet weight and cellularity were still diminished by 22% and 17%, respectively, in the adult cerebella. The results suggest that adequate postnatal nutrition will offset some, though not all of the brain biochemical changes resulting from fetal undernutrition.