Rats of Wistar strain were conceived and breast-fed until the 25th day by mothers maintained on a low protein (5%) and low caloric (21 calories/day) diet, producing a severe deficiency in weight body growth (more than 50% at the 10th day) and of the weight of the central nervous system (40% on the 15th day) both in the cerebral hemispheres and the spinal cord. Histological and biochemical analysis of the central nervous system shows: (1) Glial proliferation is insufficient and delayed, the number of glial cells is reduced by 50% on the 10th day in the cuneatus and gracilis tracts and the density of the glial cells is reduced by 50% in the corpus callosum at the 19th day. (2) Maturation of the glial cells is greatly retarded, especially in the corpus callosum a structure which matures late. On the 19th day, the majority of the cells in this structure still have a glioblastic appearence, whereas in the normal rat, at this age the majority of the glial cells are oligodendrocytes. (3) These abnormalities of glial maturation agree well with the delay of the increase of DNA, RNA and protein measured in the spinal cord and cerebral hemispheres. (4) There is a defect in myelination assessed by estimation of the density of the myelin fibres, and a definitive reduction in the caliber of the spinal tracts.