Endurance swimming times and oxygen comsumption during strenuous exercise were measured in age-paired undernourished rats. The animals received restricted or unrestricted amounts of 27, 15 or 8% casein diets for 10 weeks. Endurance times of energy-restricted animals were similar to that of controls, despite a two-fold weight difference. Endurance time of the protein-restricted group (8% casein) was longer than that of any other group, including energy-restricted groups identical in weight. Oxygen consumption of the protein-restricted group during a non-endurance swim was significantly greater than any of the other groups, but was the same as that of the other undernourished groups during the endurance swin. Oxygen consumption by all undernourished groups was higher than that of controls during the endurance swim. Results of the endurance swimming tests show that restriction of protein or energy intake may influence exercise performance differently and that differences in work performance cannot be explained by differences in body weight. Measurements of oxygen consumption during swimming suggests that the differences in performance are related to altered metabolic requirements resulting from the dietary restrictions.