Aim: To identify the antifertility effect of intermittent oral administration of tamoxifen in male rat. Methods: Tamoxifen was administered orally at a dose of 0.4 mgkg-1d-1 with an intermittent regime for 120 days. Treated and control rats were mated with cycling female rats on days 60, 90 and 120 of treatment. The mated males were sacrificed and the weights of reproductive organs were recorded, and the serum levels of LH, FSH, testosterone and estradiol estimated by radioimmunoassay. In the female rats, the numbers of implantation sites, corpora lutea, and numbers of normal and resorbed foetuses were recorded on d 21 of gestation. The potency, fecundity, fertility index, litter size and post-implantation loss were then calculated. Results: The fecundity of male rats was completely suppressed by tamoxifen while the potency was maintained at the control level. The fertility index was significantly decreased. No viable litters were sired. Post implantation loss, indicative of nonviable embryos, was observed but was not significantly increased above the control level. The weights of the testes, epididymides, ventral prostate and seminal vesicles were significantly reduced. The blood LH and testosterone levels were significantly decreased, but not FSH and estradiol. Conclusion: Intermittent oral tamoxifen administration completely suppressed the fecundity of adult male rats with reserved potency

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