Six-month-old male rats were given diets with mounting casein and wheat gluten protein concentrations from 0% to 40% and after 14 days the optimum and minimum physiological doses were determined from changes in body nitrogen, body water, body weight and protein intake. The optimum dose for casein protein was 1.54 g/d (5% protein in the diet) and for wheat gluten protein 2.10 g/d (7.5% protein in the diet). The minimum casein and wheat gluten protein doses for 180- to 194-day-old rats, determined from body nitrogen changes, were 1499 mg/d (4.86%) and 1995 mg/d (7.12%) respectively, from body water changes 1561 mg/d (5.06%) and 2027 mg/d (7.23%) and from body weight changes 1333 mg/d (4.32%) and 1951 mg/d (6.06%). It should be noted that, unlike younger animals aged 35-49, 75-89 and 120-134 days, the optimum and minimum doses for six-month-old rats were approximately the same.