Blood flow to the ovary varies dramatically in both magnitude and distribution throughout the estrous cycle, to meet the hormonal and metabolic demands of the ovarian parenchyma as it cyclically develops and regresses. Several vascular components appear to be critical to vascular regulation of the ovary. As a first step in resolving the role of the resistance arteries and their paired veins in regulating ovarian blood flow and transvascular exchange, we characterized the architecture and intravascular pressure profile of the utero-ovarian resistance artery network in an in vivo preparation of the ovary of the anesthetized Golden hamster. We also investigated estrous cycle dependent changes in resistance artery tone. The right ovary and the cranial aspect of the uterus in 26 female hamsters were exposed for microcirculatory observations. Estrous cycle phase was determined in each animal prior to experimentation. The utero-ovarian vascular architecture was determined and resistance artery diameters were measured in each animal by videomicroscopy. Servo-null intravascular pressure measurements were made throughout the utero-ovarian arterial network in 11 of the animals. Architectural data showed a complex, anastomotic network jointly supplying the uterus and ovary: resistance arteries showed a high degree of coiling and close apposition to veins, maximizing countercurrent exchange capabilities. Arterial pressure dropped below 60 percent of systemic arterial pressure before the arteries entered the ovary. Both the ovarian artery and the uterine artery, which jointly feed the ovary, showed cycle day dependent changes in diameter. Arterial diameters were smallest on the day following ovulation, during the brief luteal phase of the hamster. The data show that resistance arteries comprise a critical part of a complex network designed for intimate local communication and control and suggest that these arteries may play an important role in regulating ovarian blood flow in an estrous cycle-specific manner.