A two-generation reproductive toxicity study of methyl tertiary-butyl ether (MTBE) was conducted in Sprague-Dawley rats. Twenty-five rats of each sex (F 0 ) were exposed by inhalation to 0, 400, 3000 or 8000 ppm MTBE vapor, 6 h a day for 10 weeks prior to mating. Parental animals were then mated within groups for up to 3 weeks. Parental females were exposed during mating, gestation and lactation (starting on day 5); parental males were exposed during mating through delivery of their last litter sired. The F 1 adults were selected from the F 1 litters and were exposed beginning on postnatal day 28 for at least 8 weeks before mating to produce F 2 litters. During exposures to 3000 and 8000 ppm MTBE, group observations included hypoactivity and lack of startle reflex in parental animals from both generations. Parental animals at 8000 ppm were also ataxic. During the pre-mating period, body weights of the 8000 ppm males from both generations and the F 1 females were significantly reduced compared to control animals. Transient body weight reduction was also observed in the 3000 ppm F 1 males and females during the pre-mating period. Lactational body weights were increased in the 8000 ppm females from both generations. In the F 1 generation, increased liver weights were noted in the 3000 and 8000 ppm animals for both sexes, although histopathological examination revealed no treatment-related effects. There were no treatment-related reproductive effects noted in any of the parameters measured in this study. Offspring survival was equivalent among treated and control groups from both generations, and there were no remarkable post-mortem findings. There was, however, a significant increase in dead F 2 pups in the 8000 ppm group on postnatal day 4. The F 1 litters at 3000 and 8000 ppm had lowered body weights from postnatal days 14-21 and 14-28, respectively. The F 2 generation of pups at 3000 and 8000 ppm also exhibited lowered body weights from postnatal days 14-28 and 7-28, respectively. Body weight gains in both the F 1 and F 2 litters were also reduced for the corresponding time intervals. Thus, exposure to MTBE vapor produced no reproductive toxicity to two generations of Sprague-Dawley rats even in the presence of parental toxicity at 3000 and 8000 ppm. Postnatal toxicity was observed in the offspring of both generations, but only in the presence of maternal toxicity. The no-observed-effect level (NOEL) for both parental and postnatal toxicity is 400 ppm, and the NOEL for reproductive toxicity is at least 8000 ppm.