We measured the effect of a 4-weeks treatment with arginine aspartate on 21 athletes. By a double blind methodology, we were able to show that the treatment group had an enhanced maximal oxygen consumption as well as a significantly decreased plasma lactate concentration at a work intensity of 200, 300 and 400 W on the treadmill. By the same token, 5 and 20 minutes after completion of the test run, blood lactate levels were significantly lower in the group receiving arginine aspartate. Nevertheless, at the 4 mmol lactate threshold, power and total work output were the same for both controls and treatment groups. These results show a favourable effect of arginine aspartate on blood lactate levels.