Vaginal oxygen and carbon dioxide tensions were measured continuously in a group of normal young women on the second day of menstruation during a 90-minute period. PO2 averaged 3 mm Hg (+/- 11 SD) and PCO2 averaged 64 mm Hg (+/- 13 SD). The value rose to that of atmospheric air when a tampon was inserted and gradually fell, giving a mean value of 112 mm Hg (+/- 18 SD) during the following 90 minutes; preinsertion values were reached in about 8 hours. Carbon dioxide rose rapidly to almost preinsertion values (mean value of 50 mm Hg +/- 12 SD) during the 90-minute period and remained steady at this level during extended periods. As in vitro studies have indicated an oxygen-dependent production of a toxin-like protein from Staphylococcus aureus, it is suggested that intravaginal tampons may be a risk factor in the development of toxic shock syndrome by supplying oxygen, thus changing the vaginal microenvironment from anaerobic to aerobic.