Pregnant rats were treated with dexamethasone in drinking water (10 micrograms/ml) from the 15th to the 22nd day of pregnancy. Dexamethasone significantly depressed cell numbers in fetal rat adrenal glands, liver, lung, testes and pituitary from the 17th to the 22nd day of pregnancy. These data indicated that dexamethasone retarded cell multiplication in rat fetuses. Weight per cell and protein per cell were less in fetal adrenal glands, kidneys and pituitary after treatment with dexamethasone. Dexamethasone had no effect on RNA per cell in various fetal tissues. The nucleic acid synthesis in vitro from 3H-thymidine and 3H-uridine were reduced in all observed fetal tissues after dexamethasone treatment on the 22nd day of pregnancy. These results show that dexamethasone has a multisystemic effect on fetal rat development, depressing nucleic acid synthesis and depleting cell numbers and cell size in various fetal tissues.