The distribution and form of amoeboid microglia in the brain of neonatal rats have been studied with the light and electron microscope. With the silver carbonate method of Rio-Hortega, two major 'colonies' of amoeboid microglia are identified: (1) in the supraventricular corpus callosum in which the nerve fibres are widely spaced, and (2) at the medial angle of the lateral ventricle inferior to the corpus callosum. Scattered amoeboid cells are also seen in the cavum septum pellucidum and in the lumen of the lateral ventricle. Associated with the subependyma forming the roof of the lateral ventricle there are also numerous amoeboid cells. Ultrastructural studies show that the subependyma includes cellular elements with features intermediate between those of immature subependymal cells and full-blown amoeboid microglia. It is suggested that the latter are derived from the subependymal cells and that, once they are formed, they leave the subependyma and migrate into the corpus callosum and elsewhere. With the metallic stain, the amoeboid microglia present a wide diversity of appearances, some of which bear a close resemblance to typical microglia. It is therefore suggested that amoeboid microglia change into typical microglia. The present study clearly demonstrates that amoeboid microglia are active phagocytes. Their cytoplasm is heavily loaded with secretory granules (lysosomes) and give a positive reaction with PAS and acid phosphatase.