Subcutaneous injections in sesame oil of testosterone and estradiol-17 beta into ovariectomized and ovariectomized-hysterectomized young adult mice were followed in six hours or less by significant increases in the diameters of the ovarian and uterine veins as compared to the same vessels in sesame oil-injected controls. The circumflex iliac, femoral, and superior mesenteric veins and inferior vena cava failed to respond. The results demonstrate the specificity and speed of response of ovarian and uterine veins of mice and the control of the response by sex hormones.