Dynamic aspects of whole body protein (nitrogen) metabolism were explored in healthy young adults and elderly men and women. Measurements were made of the rate of whole body protein breakdown, with the aid of 15N-glycine, and the rate of muscle protein breakdown, as estimated from urinary N tau-methylhistidine excretion. The results also were evaluated in relation to obligatory (endogenous) urinary nitrogen losses, previously determined in this laboratory for the two age groups. Rates of whole body and muscle protein breakdown, per unit body weight, were lower in elderly subjects than in young adults. Muscle accounted for a mean of 27% of whole body protein breakdown in young adults and 20% or less (p less than 0.01) in elderly subjects. Daily obligatory N loss was positively correlated (p less than 0.01) with whole body protein breakdown. It was calculated that muscle contributed less to the obligatory N output in elderly subjects than in young adults. These results indicate a change in the distribution of whole body protein metabolism during aging in human subjects, with muscle making a lower contribution to total body protein metabolism in elderly subjects compared with young adults.