Four different diets designed to be of differing quality were fed to large groups of male weanling rats. These diets were: diet C consisting of commercial Rat Chow: diet CG, the same diet diluted with 70% glucose calories, diet A, a simulated "American" diet made up of 25 widely used foods, diet AS, the same diet supplemented with small amounts of 25 vitamins and minerals. The rats on these four diets were observed for 69 days under different circumstances and treatments to ascertain the existence of hitherto unused criteria which might advantageously be applied in nutritional studies. Among the less traditional criteria found to be significantly affected by the diet were: 1) voluntary consumption of food, 2) sleeping time after anesthesia, 3) weight gains after surgery, 4) healing time after surgery, 5) hair growth after clipping, 6) voluntary sugar consumption, and 7) recovery time after cyanide poisoning. These findings suggest that there are probably many other unexplored criteria which could be used advantageously in nutritional experimentation.