The distribution of catalase-containing microperoxisomes was studied in the central nervous system of rats during the early postnatal period when the processes of myelination and cell differentiation are active. The regions selected for study included a region previously found in adult animals to contain substantial numbers of reactive microperoxisomes, as well as areas where few such bodies were seen. Microperoxisomes were relatively numerous in all areas during the first two postnatal weeks; at later times they occurred less frequently, or, in some areas, were almost entirely absent. In early postnatal CNS, catalase-positive microperoxisomes were found in all cell types in all regions studied. Neurons of both cerebrum and cerebellum contained catalase-positive microperoxisomes during the first two postnatal weeks; few such neuronal bodies are found in these areas after the third postnatal week. During the period of active myelination, catalase-positive microperoxisomes were found in oligodendrocyte cell bodies, and also in oligodendrocyte processes associated with forming myelin sheaths in all areas. In several areas during the first 3 postnatal weeks, catalase-positive bodies were seen in synaptic terminals, a location where they are seldom observed in mature tissue. Cells of postnatal choroid plexus also were found to contain modest numbers of reactive microperoxisomes.