Pregnant NMRI-mice were X-irradiated with single doses of 0.95 Gy (100 R) and 1.9 Gy (200 R) on day of gestation (dg) 12. For sampling, anesthetized animals were perfused with buffered glutaraldehyde solution or fixed by immersion in Karnovsky solution. LM, SEM, and TEM studies were carried out on brains prenatally and up to the age of 20 months to follow the radiation effects on the developing lateral choroid plexus. Radiation-induced changes were found using all three methods and at all stages studied. The normally sickle-shaped and stretched choroid plexus is shortened and irregular, and the dome-shaped plexus cells are flattened. Their superficial fine structures, i.e., the microvilli and cilia, are altered. Three stages of severity can be distinguished and the internal hydromicro-cephalus increases from stage I to III. Intercellular spaces of the treated plexus epithelium are often dilated, but the tight junctions at the ventricular surface seem to be intact. The interstitium shows large dilations in comparison with the controls. Thus, gross changes and alterations in the fine structure can be induced in the choroid plexus by doses of 0.95 Gy and 1.9 Gy, which persist throughout postnatal life.