Oxygen consumption in maximal exercise (VO2max) was found to increase during nutritional rehabilitation of severely undernourished, sedentary adult males only when dietary protein was increased from 27 to 100 g/day. Adequate caloric intake, maintained for 45 days before the increase in protein content of the diet, had no effect on VO2max. The VO2max values were found to correlate best with the creatinine excretion rates (an index of the muscle cell mass) at the different stages of repletion. The average increment in VO2max/muscle cell mass ratio, although significant, was of a small magnitude (+15%). The VO2max/muscle cell mass ratio correlated only with the blood and plasma volumes at the different stages of repletion. After 2.5 months of protein repletion the VO2max values were still lower than those in mildly undernourished or normal Colombian adult males. Maximal endurance time at 80% aerobic load decreased significantly from an average of 113 min at the beginning of hospitalization to 42 min at the end of the period of protein repletion. This could be the result of replacing carbohydrate calories with protein in the diet, producing depletion of muscle glycogen stores, and/or to "detraining" effects associated with sedentary hospital life.