This report describes an investigation of the effect of an alteration in the amount of protein, carbohydrate and fat in hypocaloric diets on the concentration ratios of valine/(isoleucine + leucine) in the morning fasting blood in healthy men: in some cases the effect of a higher caloric diet was also analysed. During the nutrition periods, each over 10 days, the concentrations of branched chain amino acids and alanine, blood levels of insulin, glucose and free fatty acids and the nitrogen balance were estimated daily. With hypocaloric diets, as well as with hypercaloric diets, supplementation of food protein from 0.6 or 0.8 up to 2.0 g protein per kg body weight (isocalorically) induced a rise of the molar ratios of valine/(isoleucine + leucine) in blood from 0.98 to 1.25 (4600 kJ/d) or 1.06 to 1.45 (14600 kJ/d) respectively. A correlation of molar ratios of valine/(isoleucine + leucine) in the blood and the nitrogen balance could not be established. However, it is suggested that ratios above a limiting value exclude the possibility of a distinct negative nitrogen balance during 7 or 10 days of observation.