Quantitative analysis of cellular densities as well as an estimate of the cortical volume and of its total cell population were performed on the cortex of postnatally undernourished mice (2--21 days) and on that of pups from malnourished mothers (gestation and lactation). Animals were followed until 180 days of age after more than 5 months of nutritional rehabilitation, and data were obtained at 10, 30, 60, and 180 days of age. The neuronal density was much higher in all cortical layers of the two experimental series, suggesting a delay in cortical maturation. Moreover, layers II, III, and IV were more delayed than layers I, V, and VI. Postnatal undernutrition had more severe effects than maternal malnutrition and the degree of recovery after a long rehabilitation was much less. The increase of the cortical volume was greatly reduced in postnatal undernutrition and to a lesser degree in maternal malnutrition. The total number of glial cells was also reduced more in postnatal undernutrition than in maternal malnutrition, but the total number of neurons was never smaller than in the controls.