The form and function of the mesometrial smooth muscle and the interposed mesometrial branches and tributaries of the uterine vessels were studied in the nonpregnant and pregnant mouse to see whether contractions of mesometrial muscle alter uterine blood flows. Histological sections of mouse uterine horn demonstrated that the outer longitudinal layer of myometrium extends onto the mesometrium and sandwiches the mesometrium and its vessles as a bilaminar myometrial extension (BME). The BME ends midway across the mesometrium as a free edge. Cleared specimens, perfused with silicone rubber, revealed that the mesometrial branches and tributaries of the uterine vesels formed longitudinally communicating looping arcades between adjacent mesometrial vessels. Comparisons with human dissections and uterine histological sections revealed similar patterns of mesometrial smooth muscle and mesometrial blood vessele. BME activity and its control of mesometrial blood flow were studied by transillumination of the surgically exposed mesometria of anesthetized day 12 pregnant mice. Observed contractions of the BME coincided precisely with uterine contractions measured at the cervix and the BME contractions diminished or stopped venous outflow particularly in the midhhorn regions. Arterial flows seemed to be unaffected and were diminished or halted only during infrequent forceful and sustained contractions. Trapped venous outflow passed up or down the venous arcades to escape through less restricted mesometrial veins. Uterine and BME contractions normally take place throughout gestation. It is possible that abnormally long and forceful contractions may compormise the embryo or fetus.