In this study, we examined changes in vascular resistance contributing to increased ovarian blood flow in the pregnant rat. Ovarian brood flow was monitored in vivo using a venous outflow cannulation technique in nonpregnant rats and in pregnant rats at Day 16 and Day 22, and increased from 0.18 +/- 0.02 to 0.81 +/- 0.09 and 1.02 +/- 0.08 mi min(-1) ovary(-1) (mean +/- SEM; n = 7, 7, 6), respectively. Intrinsic vessels within the ovarian complex accounted for 81%, 73%, and 70% of total resistance to ovarian blood flow. Of the two major extrinsic supply vessels, from one-half to two-thirds of the ovarian blood was derived from the uterine artery, and the ovarian artery never contributed to uterine blood flow. These results indicate that the major supply vessels are unlikely to limit ovarian blood flow even near term when competing demand by the gravid uterus reaches a peak. The finding that ovarian blood flow is derived predominantly from the uterine artery may relate to local mechanisms that influence ovarian function and fetal growth.