A lectin-histochemical study on microglial development has been performed on the rat central nervous system. Isolectin B4 from the Griffonia simplicifolia (GSA I-B4) and Ricinus communis agglutinin-120 (RCA-1) were used as labelling lectins. Our results demonstrate the existence of microglial elements in the nervous parenchyma at E18, derived from the meningeal connective tissue layer. Later, another microglial source became evident, namely the cavum septum pellucidum, which serves for entrance of microglia pervading the supraventricular corpus callosum and the subependyma of the lateral ventricles. From P12 onwards, the microglial sources became inactive.