Epiplexus cells in the telencephalic choroid plexus of postnatal rats were shown with the electron microscope to be of two kinds, one with monocyte features including an indented nucleus with coarse chromatin, numerous polyribosomes, long profiles of rough endoplasmic reticulum, a well-developed Golgi apparatus, lysosomes, microtubules and coated vesicles, long cytoplasmic process and filopodia; the other with the addition of highly vacuolated cytoplasm. In order to clarify the origin of these cells, rats were given two intravenous injections of carbon particles. Shortly after the second carbon injection (1-4 days) none of the epiplexus cells were tagged with carbon. Five and six days after the second carbon injection, however, a variable number of epiplexus cells were labelled with intracytoplasmic carbon. Their number decreased later and by 9 days hardly any were seen. The present evidence suggests that the carbon-labelled epiplexus cells are derived from circulating monocytes which have ingested the carbon particles while in the blood. The labelled cells then cross the walls of subependymal blood vessels, penetrate the multilayered subspendyma and ependymal lining and enter the lumen of the lateral ventricle.