Oxygen tension was measured using flexible polarographic microelectrodes within the oviductal and uterine lumen in rhesus monkeys ( n = 9), golden hamsters ( n = 21) and rabbits ( n = 6), during the reproductive cycle (monkey), during oestrus and pseudopregnancy (hamsters, rabbits) and during pregnancy (hamsters). In general, oxygen tensions in each species were much less than half of atmospheric O ₂ , ranging from high values of about 60 mm Hg (8.7% O ₂ ) in the rabbit oviduct, rabbit and hamster uterus, to as low as 11 mm Hg (1.5% O ₂ ) in the monkey uterus. Oxygen tensions did not vary significantly between left and right sides of the reproductive tracts (all species), nor between pregnant and pseudopregnant states nor between oviduct and uterus (hamsters). Differences owing to reproductive stage were found in the monkey oviduct, hamster oviduct and uterus, and rabbit uterus. Oxygen tensions were consistently very low (11–14 mm Hg) in the monkey uterus throughout the menstrual cycle. In hamsters and rabbits, intrauterine O ₂ decreased significantly at about the normal time of blastocyst formation and implantation, to 37 mm Hg (5.3% O ₂ ) and 24 mm Hg (3.5% O ₂ ), respectively. This study indicates that embryos develop in vivo under low oxygen concentrations, especially during the peri-implantation period. The data have implications for investigations of embyro metabolism and for improving embryo development in vitro .