Eight young adult men were used to investigate the metabolic effect of controlled 9-day periods of dietary conditioning on subsequent 5-day periods of caloric restriction (stress periods). Half of the subjects were conditioned with diets high in fat (about 68 per cent) while the others had diets low in fat (about 10 per cent). Each day of the stress periods a 148-gram portion of pemmican (about 1000 calories) was ingested by each subject. The criteria used to follow the metabolic response of the subjects were blood glucose levels, excretion rates of urinary ketone bodies, and nitrogen balances. It was found that the change in the average blood glucose level from the conditioning periods to the stress periods was not as drastic for the low fat conditioned men as for the men conditioned on diets high in fat. Neither the excretion rates of urinary ketone bodies nor the nitrogen balances were improved appreciably during the stress periods by either type of dietary conditioning. Comparisons based on body size showed that the smallest men of the group had higher blood glucose levels and lower excretion rates of urinary ketone bodies in the second stress period than did the larger men. The catabolism of tissue protein did not appear to be related to body size.