AbstractMaternal administration of cadmium (Cd) has previously been shown to cause reduced lung weight and altered pulmonary surfactant accumulation in fetal rats. This leads to neonatal respiratory distress. In the present study the effects of Cd on the ultrastructural development of alveolar epithelium was studied to detect any alterations which could contribute to neonatal respiratory difficulties. Pregnant rats received 8.0 mg/kg body weight of CdCl2sub‐cutaneously on days 12–15 of gestation. Fetal tissue was examined by electron microscopy throughout the period of alveolar cytodifferentiation. The undiffer‐entiated epithelium of both treated and control fetuses was composed of simple columnar cells with apical and basal glycogen deposits. Cytodifferentiation in controls consisted of a gradual flattening of alveolar type I cells and the appearance of surfactant‐containing lamellar bodies in alveolar type II cells. Cytoplas‐mic glycogen was prominent up to day 20, then diminished to low levels on day 21. Epithelial differentiation in treated fetuses was delayed in most alveoli. On the two days before term some alveoli appeared normal, but many were retarded or arrested in their development. Cellular glycogen pools were reduced in treated alveoli and were absent just prior to birth. Lamellar bodies in treated alveoli were less numerous, correlating with the diminished pulmonary surfactant lecithin content reported earlier. These alterations in prenatal ultrastructural maturation could contribute to the postnatal respiratory difficulties previously