Uterine vascular architecture through estrus cycle of the rat was investigated by the microcorrosion casting/scanning electron microscopy method. Arterial system in the uterine horn was divided into two types, one of which consisted of circumferential artery with its branches extending to both side walls and an antimesometrial wall. The other system consisted of arterioles which turned into the vascular congromerate in the mesometrial triangle and then distributed their branches to the mesometrial wall. Both of two kinds of arterioles, the terminal arteriole in the myoendometrium and the larger arteriole composing the vascular congromerate, showed the circular impressions around them. Arteriovenous anastomosis was not observed in the uterus. During proestrus and estrus, uterine blood vessels were lengthened except for arterioles in the vascular congromerate, and the subepitherial capillary network became to be filled completely with resin and showed the signs of leakage. These changes reached the maximum in estrus, then subsided during the later part of the cycle. These observations suggest that different types of arterioles in the uterus have their own sphinctors, and maximum enlargement of the uterine vascular bed occurs in estrus concomitantly with increased capillary permeability.