In this short review, selected aspects of body protein and amino-acid metabolism during aging in human subjects have been explored. There is a progressive diminution of total body protein with aging, due largely to a loss of skeletal muscle protein. These changes are accompanied by a shift in the overall pattern of whole body protein synthesis and breakdown, with muscle mass estimated to account for about 30 percent of whole body protein turnover in the young adult, as compared with a lower value of 20 percent or less in elderly subjects. There is no evidence that the loss of muscle protein is related to a reduced "amino acid tolerance" to insulin, since leucine-insulin metabolic relationships appear to remain intact during advancing adult age. However, because skeletal muscle mass plays an important role in the response to body protein and amino-acid metabolism to stress, such as that due to infection or trauma, this decline in the contribution of muscle to total body protein metabolism might be a factor responsible for the reduced ability of older people to withstand such unfavorable circumstances. Recent observations on the importance of muscle as a source of glutamine that is essential for immune cell function support this contention. The dietary requirements for individual essential and for total protein are limited and often contradictory (see, for example, Young, et al, 1982). However, elderly individuals are more likely to be affected by various biological, environmental and social factors, which would generally increase protein and amino acid needs above those for younger adults. Thus, in practice, the protein needs in the elderly are likely to be higher than those in the young. The decline in energy intake, together with its possible consequences for reducing the efficiency of dietary protein utilization, also will tend to increase the protein need for elderly subjects, relative to that for physically more active young adults. Until more data become available, it is recommended, for food planning purposes, that an appropriate protein allowance would be about 15 percent of the total energy intake, for mixed food protein sources characteristic of the diets of industrialized countries. Epidemiological surveys of the protein nutritional status of healthy, free living elderly subjects and of institutionalized individuals without clinically apparent terminal or wasting illness are consistent with this recommendation.