Summary In earlier studies, we have established thatl-arginine enhances motility and metabolic rate in spermatozoa of goat, bull and mouse. In the present study this work was extended to human sperm cells obtained from the semen samples of asthenospermic patients, which are characterised by low motility. The metabolic rate was followed by monitoring the glucose consumption (1-13C glucose as substrate) and the production of lactate in sperm cells, using 13C NMR. The stimulatory effect ofl-arginine was neutralised on adding an NO-synthase inhibitor like Nω-nitro-l-arginine methyl ester. On the other hand, the inactived-enantiomorph did not affect the stimulatory effect ofl-arginine. This strongly suggests thatl-arginine acts through the NO signal pathway. We also demonstrated that the stimulatory effect ofl-arginine was inhibited in the presence of anion channel inhibitors like 4-acetamido-4′-isothiocyanostilbene-2,2′-disulphonic acid, 2,4-dinitrophenol and carbonyl cyanide m-chlorophenylhydrazone. Furthermore, bicarbonate supplementation was found to be essential for the action ofl-arginine. These observations indicate thatl-arginine induces NO synthesis and stimulates motility and metabolism only when an active anion transport system is present.