The effects of estradiol, estrone and estriol on the tonus of the isolated vascular segments of human umbilical artery were studied by a perfusion technique. Estradiol (10 microgram/ml), estrone (10 microgram/ml) or estriol (2 and 10 microgram/ml) produce relaxation of the vascular bed which was reflected in a significant reduction in basal pressure of segments perfused at constant rate. A dose-dependent relationship was demonstrated for estriol. The time dependence and extent of relation were similar for all three natural estrogens.