Introduction: Genistein is a plant-derived estrogen-like that has multiple effects, including its ability to ameliorate endothelial dysfunction in ovariectomized rats. However, genistein can also inhibit tyrosine protein kinases resulting in inhibition of angiogenesis. Our objective was to study the effects of genistein on the amounts and function of en- dothelial cells of ovariectomized rats. Methods: Twenty female mature Wistar rats underwent bilateral ovariectomy. Four groups of ovariectomized rats were formed, three groups treated with genistein doses of 0.1 (low), 0.5 (moder- ate), and 1 mg/kg body weight (high), respectively orally daily four weeks after ovariectomy, and the ovariectomized rats without treatment represented are a positive control group. Five female rats without ovariectomy and genistein supplementation were the normal groups. The endothelial cell number was measured using hematoxylin-eosin stain- ing, and endothelial cell function was evaluated through the vasodilatory level in endothelium-intact isolated rat aortic rings. The data were analyzed using an LSD posthoc test with a significant difference at p<0.05. Results: The endothelial cell number and endothelium-dependent vasodilatation of ovariectomized rats supplemented with genistein in low and moderate doses increased significantly compared with the positive control group, but it was still significantly different from the normal group. In contrast, both variables in the group treated with genistein in high doses have no significant difference (from ovariectomized rats. Conclusion: The study shows that genistein has a biphasic response to ovariectomized rat endothelial cells, in number also its function.