Weanling and adult male rats were offered pairs of diets containing either 0 and 20%, 0 and 40%, 0 and 60%, or 10 and 40% casein. Initially they preferred the lower protein diets, and then after one to ten days abruptly switched to adequate levels of protein choice. After 14 days, the diets were changed so that the first group received 10 and 40%, the second 20 and 60%, the third 10 and 40%, and the fourth 0 and 60% casein. Following the change, all weanling rats showed significant and systematic shifts in percentage of total energy chosen as protein (% P−E); most adults did not. Within each group, the variability in % P−E selected between different rats was higher than the day-to-day variability of individuals. In the adults no significant correlations were observed between protein selection and brain serotonin metabolism. We conclude that protein intake in rats is regulated in the sense that all animals learnt to eat sufficient protein to maintain growth, and most animals ate a constant amount of protein each day. On the other hand, the range of protein intake between individuals, and the shifts in selection among the weanlings when diet choices were changed seem to preclude the existence of a mechanism which precisely regulates protein intake.