Four trihalomethanes were administered by gavage to Sprague-Dawley rats from day 6 to day 15 of gestation. Chloroform (Ch) was administered at levels of 100, 200 and 400 mg/kg and bromoform (Br), bromodichloromethane (BDCM) and chlorodibromomethane (CDBM) were administered at levels of 50, 100 or 200 mg/kg/day. A separate control was used for each compound. Maternal weight gain was depressed in all groups receiving Ch and at the highest dose levels of BDCM and CDBM. Ch administration caused decreased maternal hemoglobin and hematocrit values at all dose levels and also produced increased serum inorganic phosphorus and cholesterol at the highest dose. Liver enlargement was observed at all dose levels of Ch but in no other treatment groups. Evidence of a fetotoxic response was observed with Ch, CDBM and Br but not BDCM. No dose-related histopathological changes were observed in either mothers or fetuses as a result of treatment. None of the chemicals tested produced any teratogenic effects.