Mated CD Sprague-Dawley rats and CD-1 mice were exposed during the period of organogenesis to target concentrations of 0, 250, 1000, and 2500 ppm methyl t-butyl ether (MTBE). None of the control or test-group animals died during the treatment or posttreatment periods. Females were sacrificed on d 20 (rats) or d 18 (mice). No adverse effects of treatment were reflected in maternal parameters of body weight, water consumption, or liver weight or in physical examination data for either species. Food consumption fell in the groups of treated rats during d 9-12; similar but nonsignificant effects were observed for mice during d 12-15. In rats, no treatment-related changes were recorded in the uterine implantation data, fetal size parameters, or fetal sex distribution data. Examination of fetuses for external abnormalities, skeletal malformations, or ossification variations did not reveal any changes caused by MTBE exposure. A slight increase in fetal resorptions was observed in the groups of mice exposed to low and high concentrations; this increase was attributed to two females in each group that had an unusually high number of resorptions, rather than to the treatment itself. No significant effects were observed in any groups of treated mice on external and soft-tissue examination or evaluation of skeletal abnormalities or ossification variations. The incidence of fused sternebrae in the high-concentration group increased slightly, which might be attributed to fetotoxicity.