Some effects of undernutrition were noted in the brains of 21-day-old pups undernourished from birth by being reared by nursing dams fed on a low protein diet. As compared to the normally fed control pups, the body weight, brain weight, and thickness of the parietal neocortex and paravermal cerebellar cortex were significantly lower in the undernourished pups (p less than 0.001), while the thickness of the cortices of the dentate gyrus and hippocampus CA3 only reached significance at the 5% level. Again, as compared to the control pups, the number of spines per unit length on the distal dendritic segments of the pyramidal cells of lamina III of the parietal neocortex and of the hippocampus CA3 reached significance at the 5% level whereas that of the granule cells of dentate gyrus and Purkinje cells of the paravermis did not reach significance at the 5% level. The myelination of the pyramidal tract and spinal trigeminal tract in the lower medulla was less advanced in the undernourished pups. To date, the demonstration of precise functional correlates for the observed changes of morphological and physiological parameters in animals subjected to undernutrition has been elusive. An approach that may lead to unraveling this problem is suggested in a discussion of relating sources of input to the specific dendritic segments where the spine counts were made.