The epiplexus cells in the lateral ventricles of the adult monkey (Macaca Fascicularis) were examined in the electron microscope. The majority of the cells displayed ultrastructural features typical of tissue macrophages, while others resembled monocytes or microglial cells. These cells were characterized by an abundant cytoplasm containing vacuoles, a prominent Golgi complex and a variable number of electron-dense granules resembling lysosomes. The cell surface often showed pseudopodial or filopodial processes. Macrophages bearing similar features were also observed in the connective tissue stroma of the choroid plexus. In the same area, occasional circulating monocytes, which appeared to be in the process of migrating out of the vascular lumina, were seen. Furthermore, the present study showed the frequent occurrence of macrophages which appeared to be within the choroidal epithelial cells. Based on these observations, it was suggested that the epiplexus cells were derived from diapedesis and metamorphosis of the circulating monocytes in the core of the choroid plexus. It would appear, then, that the extravasated cells crossed the choroidal epithelium by an intracellular pathway to reach the ventricular lumen to become the macrophagic epiplexus cells. However, an alternative route of cell migration, i.e. through the choroidal epithelium by an intercellular pathway, was also considered.