Aim: To examine the effect of L-arginine, the substrate of nitric oxide (NO) synthase, on reproductive function ofmale rats. Methods: Male rats were gavaged with either L-arginine (100 or 200 mg·kg~(-1)·d~(-1)), D-arginine (200mg·kg~(-1)·d~(-1)) or vehicle (0.9% NaCl) for seven consecutive days. Their sexual behaviour and fertility were evaluat-ed using receptive females. Results: L-arginine (200 mg/kg) had no significant effect on sexual competence (interms of sexual arousal, libido, sexual vigour and sexual performance). In mating experiments, the higher dose of L-arginine effectively and reversibly inhibited fertility, whilst the lower dose and the inactive stereoisomer D-arginine hadno significant effect. The antifertility effect caused by L-arginine was due to a profound elevation in the preimplantationloss mediated possibly by impairment in epididymal sperm maturation, hyperactivated sperm motility and sperm capaci-tation. Conclusion: Elevated NO production may be detrimental to male fertility Aim: To examine the effect of L-arginine, the substrate of nitric oxide (NO) synthase, on reproductive function of male rats. Methods: Male rats were gavaged with either L-arginine (100 or 200 mg · kg -1) (-1)), D-arginine (200 mg · kg -1 d -1) or vehicle (0.9% NaCl) for seven consecutive days. Their sexual behavior and fertility were evaluated at-ed In mating experiments, the higher dose of L-arginine was effectively and in the same way that L-arginine (200 mg / kg) had no significant effect on sexual competence (interms of sexual arousal, libido, sexual vigour and sexual performance) reversibly inhibited fertility, whilst the lower dose and the inactive stereoisomer D-arginine hadno significant effect. The antifertility effect caused by L-arginine was due to a profound elevation in the preimplantation loss mediated by impairment in epididymal sperm maturation, hyperactivated sperm motility and sperm capaci-tation. Conclusion: Elevated NO production may be detrimental to male fertility