In several earlier single generation reproductive toxicity studies, rats were exposed to high concentrations of octamethyleyclotetrasiloxane (OMCTS) for a prolonged premating interval (28 - 70 consecutive days). Exposures continued during the mating period, throughout gestation, and (in some studies) during lactation. In the earlier studies, decreased litter sizes. decreased numbers of uterine implantations. and reduced numbers of corpora lutea were seen; these findings were previously communicated to EPA. This study was conducted to help identify the sensitive interval and reproductive events affected in female rats exposed to OMCTS by whole body vapor inhalation. Female rats were exposed for six hours daily to the test article at different times during their reproductive cycles as noted below. Lapohyster- ectomies were performed on gestation day 20, and corpora lutea and uterine contents were evaluated.