The effect of a low calorie (1100 kcal), low protein (35 g) intake for 9 days on nutritional and laboratory parameters was studied in six young and six elderly healthy subjects. All subjects lost weight on the test diet (an average of 2.0 kg in the young and 2.1 kg in the elderly). Basal serum albumin, serum transferrin, creatinine height index, and creatinine clearance in the elderly subjects were lower than in the young (p less than 0.05). Serum albumin remained unchanged at the end of diet in both groups. There was, however, a significant decrease in the transferrin level in the younger subjects (p less than 0.01). The decrease in creatinine height index was not significant in either group. Baseline nitrogen balance of the elderly was -1.1 g/day compared to +0.5 g/day (p less than 0.01) in the young subjects. The test diet caused a negative nitrogen balance of about -5 g/day in both groups (p less than 0.005). Blood urea nitrogen was significantly lower at the end of the test diet and was related to the intake of protein in both groups. At the end of the diet the serum potassium and GGT were significantly lower in the young (p less than 0.01); serum creatinine was unchanged but creatinine clearance was significantly lower in both groups. From this study it appears that significant changes occur in nutritional and laboratory parameters within 9 days on a diet deficient in calories and protein in normal healthy subjects. These changes need to be recognized as having nutritional basis and should not be attributed to illness or drug therapy.