Some evidence suggests that the uterine tube plays an active role in supporting the development of the preimplantation embryo. To determine whether there were morphological and/or ultrastructural changes in the uterine tube as a consequence of pregnancy, the region of the uterine tube containing 2-cell embryos or oocytes was examined in untreated d 2 pregnant and pseudopregnant mice. The general morphology of the uterine tube was assessed by light and electron microscopy. Qualitative assessment of uterine tube morphology suggested that although the epithelium and muscular layers of the oviduct were similar in pregnancy and pseudopregnancy there were differences in the vasculature between the 2 groups. In transverse sections of the uterine tube, cross-sectional vascular surface area was assessed morphometrically. There was a statistically significant decrease (P < 0.001) in the cross-sectional vascular surface of the oviduct in pregnant mice compared with pseudopregnant mice. This reduction was apparently due to (1) a decrease in the median cross-sectional surface area of small intramural blood vessels, and (2) an apparent collapse of many subepithelial capillaries. Pregnant mice also had fewer fenestrated capillaries whereas such vessels were common in pseudopregnancy. Activated, aggregated platelets were only observed in the capillaries and venules of pregnant, untreated mice. Some animals were treated with 10 micrograms WEB 2086/day on d 1 and d 2, a platelet-activating factor receptor antagonist. This treatment reversed the reduced vascular surface cross-sectional area found in early pregnancy, but had no effect on vascular measurements in pseudopregnancy.