REview of studies on urinary excretion of proteins, electrolytes, simple compounds and hormones from the point of view of how changes occurring in the excretion of these metabolites relate to fatigue. Mainly due to individual variability, no consistent relationship has been established between the excretion of any of these substances and fatigue. Other variables causing inconsistent results are circadian rhythm, food and fluid intake, the volume of excreted urine and of sweat, climatic conditions, the time intervals between taking urine samples, and the subject's state of acclimatization and training. A reduction in excretion after repeated exposures to a stress may be due either to adaptation or to fatigue. By measuring simultaneously the urinary excretion of most of the known hormones it has been established that the organism's response to stress involves the total neuroendocrine apparatus. Future research will determine whether the urinary excretion of mucoprotein or any other single substance indicative of the rate of catabolism or anabolism will give more consistent information on the intensity of stress exposure and the resultant physiological strain as well as on fatigue.