Objective: Studies show that estrogens can influence reproductive system differentiation. The main aim of this study was to determine pathological effects of estradiol valerate on testis histology and morphology in male rats. Method: Adult male albino Wistar rats were divided to control and estradiol valerate (200 μg/kg/day) receiving groups. Estradiol valerate was applied subcutaneously. After 4 weeks, testes were excised and studied morphologically and histologically. Data were statistically analyzed and compared between groups using ANOVA. Results: Our findings revealed that estradiol valerate injection resulted in reducing of testes weight and size (P<0.05). Semeniferous tubules were apparently deformed in estradiol valerate receiving animals and cellular density was also reduced. Number of spermatocytes, spermatids and sperms was decreased in estradiol valerate receving rats compared with control animals (P<0.001).