1. Whole-body protein turnover was measured in rats by constant infusion of 15N-labelled glycine, aspartate, valine and leucine and measuring the enrichment of hepatic and renal urea and ammonia nitrogen.
2. The values obtained with [15N]glycine were comparable with values reported with methods based on different assumptions.
3. [15N]Aspartate gave rise to an increased enrichment of urea and ammonia and hence to lower protein-turnover rates.
4. [15N]Valine and [15N]leucine gave low enrichments of nitrogenous end products and hence to high protein-turnover rates.
5. All 15N-labelled amino acids are not equally suitable for measuring whole-body protein turnover by the end-product method. The relative amounts of 15N going to the end products can be predicted from the known individual metabolism of aspartate and the branched-chain amino acids.