In earlier papers in this series, we have reported investigations of various phases of the metabolism of undernourished children.1 In this paper, we give the results of fifty-nine sets of experiments on the nitrogen metabolism of ten normal and thirty-nine underweight children, most of whom have served as subjects in our previous studies. Little literature is available on this subject. The only article which gives data having a direct bearing on our problem is that of Fleming and Hutchison,2 who, in a study of the metabolism of undernourished infants, reported that the normal absorption of nitrogen is from 80 to 95 per cent of the intake and that a difference was not found in the percentage of nitrogen absorbed in undernourished infants. These investigators also found that with improvement in the nutritional state, an increase in the output of urinary nitrogen occurred. In other words, as the infant