The effect of estrogen receptor blockage by Tamoxifen on testicular function were evaluated in male albino rats. Tamoxifen treatment in two doses (100 and 400 ug/d) and for different periods (10, 20, 60 and 90 days) resulted in an increase in FSH level (after 10 and 20 days of treatment), which return back to the control level after 60 and 90 days. Serum LH and testosterone and testicular testosterone level are all decreased by tamoxifen treatment. The decreased serum LH suggests that Tamoxifen may act as estrogen agonist at the hypothalamo-pituitary axis, inhibiting pituitary LH secretion. Tamoxifen decreased both serum and testicular testosterone level and altered testicular weight and spermatogenesis, resulting in decreased sperm count and increased sperm abnormality and occasional damage. We can conclude that endogenous male estrogen is an important male hormone and estrogen receptor blockage by Tamoxifen decreased male fertility.