The relative numbers of oligodendroglia were compared in representative brain regions of 21 day old undernourished and control rats. As a result of postnatal undernutrition which produced half normal body weights and a 10-15 percent reduction in brain weight, the relative numbers of oligodendroglia were slightly increased in photomicrographs of corticospinal tract (a motor tract), medial lemniscus (a sensory tract), red nucleus (a motor nucleus) and somatosensory cortex. Relative numbers were reduced in the corpus callosum, and the thickness of the corpus callosum was significantly reduced. Cell sizes of oligodendroglia were essentially normal throughout the brain, although some reductions of 5 to 6 percent were observed. Areas of brain structures in cross section were essentially unchanged. We have previously hypothesized that nutritionally induced brain hypomyelination results from a reduction in the specific numbers of oligodendroglia and consequently a lasting reduction in the brain myelin concentration. The present results are inconsistent with this hypothesis, as both the density of oligodendroglia and sizes of brain regions are essentially normal. We know from prior work using the same model of nutritional deprivation that myelin synthesis is greatly reduced. Consequently an important depressant effect of undernourishment on oligodendroglia in the developing brain involves either the communication between axons and oligodendroglia leading to myelin induction or the synthetic capacity to make myelin.