Mice (CD-1 strain) were placed on diets containing either municipal drinking water (Durham, North Carolina) or water that had been distilled and passed through cartridges to reduce organics and remove inorganics. After a two-week acclimation period, animals were bred and pregnancy confirmed by the presence of a sperm plug. During the 8-month course of the study, approximately 500 pregnant mice were sacrificed on day 18 of gestation and their fetuses examined for visceral and skeletal anomalies. No significant water-related effects were found on any fetal parameter studied except for a 28.1 percent incidence of supernumerary ribs in the tap-water group as compared to 21.1 percent in the purified-water group. No differences were noted in the type or occurrence of anomalies between the two groups. A month to month variation was observed in a number of parameters. The degree of variation was similar for the treatment groups, suggesting that these changes might be random fluctuations.