The vascular and microvascular anatomy of the rat uterus during the oestrous cycle was studied using the corrosion vascular casting/scanning electron microscopy method. Particular attention was paid to identifying each stage of the oestrous cycle prior to casting. The optimum viscosity range for the methyl methacrylate-based casting medium was between 2.3 and 4.0 centistokes; the perfusion pressure was adjusted to give an abdominal aortic pressure of 90 mm Hg. Casts show a complete replication of all uterine vessels. The longitudinal muscle capillaries of the myometrium appear thicker and more closely spaced than those of the circular muscle. Capillary networks surrounding the endometrial glands are continuous with the complex subepithelial capillary plexus. The uterus undergoes large variations in diameter during the oestrous cycle, and the corresponding changes in the spatial arrangement of its vasculature are demonstrated. No new vessel growth was detected during the cycle. The vaso-activity of methyl methacrylate makes vessel diameter measurements from the casts unreliable. This and other problems relating to the physiological interpretation of corrosion casts are discussed in detail. This work provides a thorough description of the morphology and unique structural variability of the rat uterine vasculature during the oestrous cycle.