The effects of a low protein diet during gestation, lactation, and after weaning on the anatomical development of a subcortical nucleate formation, the neostriatum, and a reticulate formation, the diagonal band of Broca, were studied. At 10, 30, and 90 days the volume of the neostriatum was decreased in the experimental rats. However, the percent of the brain volume that was neostriatum was unaffected at each of these ages. In a rapid Golgi study of individual neurons at 90 days of age there was no significant effect of the low protein diet on the dendritic length of three different types of neurons within the neostriatum. However, its heavily spined dominant neuron showed a significant decrease in synaptic spine density. In the reticular formation, there was also no significant effect on dendritic length. A minute, apparently axonless cell corresponding to the neurogliaform cell of Ramón y Cajal, showed a decrease in extent of its cell processes only in the neostriatum. When compared to cortical formations, these phylogenetically more conservative neuronal substrates appear to be more resistant to the effects of undernutrition.