Various doses of methylmercuric chloride (MMC) were administered orally to pregnant Fischer 344 rats on d 7 of gestation. On d 20 of gestation the dams were laparotomized under ether anesthesia, and the fetuses were removed. Maternal body weights were decreased for 2 d and 6 d in rats given 10 and 20 mg/kg MMC, and were continuously decreased for those given 30 mg/kg MMC. Maternal weight gain of each group was decreased to 86.2%, 78.9%, and 61.9% of control group on d 20 of gestation. The reduction of litter weight was greatly enhanced with increasing MMC doses, presumably due to postimplantation loss, which was already increased at high treatment levels. The LD50 of MMC for fetuses was determined to be 16.5 mg/kg. Mercury content in maternal organs was highest in kidney, followed by blood, spleen, liver, and brain, while in fetal organs it was highest in liver. Fetal liver and brain contained more mercury than maternal liver and brain. However, fetal kidney retained less mercury than maternal kidney. Fetal ossification center was not completely formed in sternebrae, particularly in fifth and second bones, pelvic bones, and pectoral phalanges of fetuses in rats treated with 30 mg/kg MMC. The ossified lengths of skeletal bone stained with alizarin red S were developed least in fifth sternebrae, metacarpals in the pectoral girdle, and ischium in the pelvic girdle, and were severely retarded in development as position of the ribs goes from the sixth bone (center) to the first and 13th bone (each edge). These results indicate that MMC is embryotoxic in Fischer 344 rats.