The development of amoeboid microglial cells was studied in embryonic rats extending from the twelfth day post-conception (E12) through late gestation. With the silver stain, amoeboid microglial cells in the supraventricular corpus callosum appeared at around E17 and their number increased dramatically to more than 20 times just before birth. Most of the cells were round with projecting fine cytoplasmic processes. The silver-stained preparations also showed that the supraventricular region was vascularised around E13. With the same method, it has been shown that islands, with an admixture of erythroblasts and amoeboid microglial cells, were a common feature in the embryonic brains. Electron microscopic study confirmed the existence of amoeboid microglial cells in E15-E18 rats. Among these, there were small dense cells with little cytoplasm containing predominantly free polyribosomes. At the cell surface, there were microvilli. Other cells were large and appeared to be actively involved in phagocytosing degenerating cells. Transitional forms were present. With growth, the amoeboid microglial cells further differentiated and their cytoplasm accumulated abundant lysosomal granules. Cytochemical study showed that amoeboid microglial cells in the embryonic brains were stained for non-specific esterase. The observations in the present study suggest that amoeboid microglial cells are formed following vascularisation of the brain tissues. The invading mesenchymal cells with haemopoietic potentiality not only develop into endothelial cells but also into extravascular small amoeboid microglial cells which have the features of monocytes, and which, with growth, will transform into the large macrophagic cells.