The dynamic aspects of protein metabolism in uremic children were studied by using newly developed gas chromatography and mass spectrometry micromethods for determining nitrogen-15 enrichment in plasma lysine during a continuous i.v. infusion of 15N-lysine. Protein flux in uremic children was low and varied directly with protein and energy intake, which themselves were closely related in the diets consumed by the study subjects. Hemodialysis did not alter acutely protein flux. Protein flux in children undergoing chronic hemodialysis was still reduced below normal but was higher than that in nondialyzed uremic children at each level of protein-energy intake. The ratio of protein flux to protein intake was 0.05 +/- 0.003 g of protein x kg-1 x day-1 x kcal ingested in the nondialyzed uremic children, but it increased to a normal vaue of 0.11 +/- 0.03 g x kg-1 x day-1 x kcal in those on longterm hemodialysis. We conclude that the low protein turnover rates in our uremic population reflect the decreased protein-energy intake commonly found in such patients and that reduction of azotemia by chronic hemodialysis may improve whole body transport at a given energy intake.