Aim: To examine the effect of L-arginine, the substrate of nitric oxide (NO) synthase, on reproductive function of male rots. Methods: Male rats were gavaged with either L-arginine (100 or 200 mg@ kg- 1@ d-1), D-arginine (200 mg@ 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 (in terms 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 had no significant effect. The antifertility effect caused by L-arginine was due to a profound elevation in the preimplantation loss mediated possibly by impairment in epididymal sperm maturation, hyperactivated sperm motility and sperm capaci ration. Conclusion: Elevated NO production may be detrimental to male fertility.