Intrauterine oxygen tension was measured in vivo in the adult virgin rat using an oxygen microelectrode and amplifier assembly coupled with a continuous recording system. Intermittent fluctuations in pO₂ with varying degrees of regularity were observed. In the uterus of the intact rat the frequency of these alterations and level of average pO₂ were found to be determined primarily by the patency of the uterine arterioles which supply the capillary network in the endometrium. To only a minor degree did contractile activity of the myometrium affect intrauterine oxygen tension. The topical or intravenous administration of epinephrine hydrochloride or acetylcholine chloride induced vasoconstriction of the arterioles and a decrease in intrauterine pO₂. Upon the restoration of blood flow accompanying vasodilation, oxygen tension increased. The topical application of histamine phosphate inci-eased capillary bed engorgement and increased intrauterine pO₂. Following ovariectomy during estrus, average intrauterine oxygen tension increased markedly above estrous levels during the subsequent 5 days. The daily sc injection of estrone (0.5, 1.0 or 5.0 μg) prevented the post-ovariectomy elevation in average pOs, but had little effect on the rate of pO₂ fluctuation. The administration of progesterone (1.0, 2.0 or 4.0 mg) prevented in part the increase in oxygen tension following ovariectomy, but tended to enhance the rate of fluctuation. The administration of 1.0 μg estrone and 2.0 mg progesterone together resulted in an average intraluminal pO₂ level intermediate to and a frequency greater than that obtained with either ovarian steroid alone. It was concluded that the availability of oxygen within the uterus probably is regulated by action of the ovarian steroid hormones on vascular contractility and on metabolic and morphologic changes which affect oxygen availability. (Endocrinology83: 691, 1968)