Pregnant rats received a single administration of 0.5, 1.0, or 2.0 mg/kg of cadmium on day 18 or 20 or gestation. Maternal animals were killed on day 21 and samples of the caudate nucleus from fetal brains were examined using the transmission electron microscope. A 2.0 mg/kg dose of cadmium administered to maternal rats on day 20 of gestation caused the formation of vacuoles in the endothelial cells of capillaries in the fetal brain. Significant endothelial cell vacuolization was not observed in the brain of fetuses from other treatment groups. The vacuoles occurred singly, were spherical in shape, were located adjacent to the intercellular junctions and caused focal distortion of the endothelial cell. Vacuoles were the only ultrastructural alteration observed in the caudate nucleus of fetal brains.