The oxygen tension in the oviductal fluid of adult female rhesus monkeys was measured by means of miniaturized, flexible, oxygen probes between days 5 and 22 of the menstrual cycle. In the follicular phase, the value was very low, less than 10 Torr. At the time of ovulation to about 5 to 7 days thereafter, the oxygen tension in the tube on the side of ovulation increased within approximately 2 minutes after insertion of the oxygen probe from the low level to the same value as in the blood. In the contralateral oviduct, the oxygen tension remained at less than 10 Torr. These findings suggest that, in the absence of ovulation, the blood-oxygen supply of the oviduct provides the exact oxygen requirement of oviductal tissue, with no excess oxygen available for the tubal fluid. At and shortly after ovulation, the presence of a probe stimulates, by an unknown mechanism, an increase in the oxygen tension in the oviductal fluid which provides an environment sufficiently well-oxygenated for gamete survival, fertilization, and subsequent further development of the embryo. We suggest that the ovum entering the oviduct provides a similar stimulus with a similar result by the same unknown mechanism.