Although fatigue is a well-known phenomenon and the phrase" exercised until exhaustion" is commonly understood, there is no unequivocal agreement on the fundamental nature of the fatigue process. Ammonia was linked to the development of fatigue as early as 1922, when ammonia production was observed from stimulated nerve and the question whether there could be a relationship between ammonia production and the muscle activity was raised. The immediate source of ammonia from muscle appears to be a result of the deamination of AMP and is more apparent in fast-twitch than in slow-twitch fibers. More recently, increases in blood ammonia levels have been reported in rats after swimming and in humans after arm work, maximal cycle ergometry, and treadmill exercise. Elevated blood ammonia has also been linked to a surprising variety of functional and metabolic neurological disturbances other than exercise and fatigue, including the development of hepatic coma, convulsions from ammonia toxicity precipitated by high-pressure oxygen breathing, epileptic seizures, and decreased neuronal excitability. In addition, a number of genetic disorders (inborn errors in metabolism, or IEMs) are characterized by elevated blood ammonia concentrations. Symptoms of neural disability in all of the above conditions have been related to the concentration of ammonia in blood. Although these studies do not relate to exercise or fatigue directly, it is conceivable that our understanding of the effect of high concentrations of blood ammonia in these clinical conditions may provide valuable insight into the effect of ammonia during exercise. This paper reviews the effect of ammonia production during exercise and other conditions upon purposeful activity and the development of fatigued states.