1. Activities of arginine synthetase system enzymes, alanine aminotransferase (AlT) and branched-chain amino acid transaminase (BATase) were measured in control and uraemic rats after 2 weeks on a free protein intake and after 10 days on a low-protein diet.
2. Protein restriction alone reduced the activity of all three enzymes.
3. Uraemia alone increased the activity of arginine synthetase system enzymes and AlT. BATase was not altered.
4. After protein restriction the BATase activity in uraemic rats decreased to the same extent as in controls. The activity of arginine synthetase system enzymes and AlT were significantly higher than in protein-restricted controls and were the same as in control rats on a normal diet.
5. Practical consequences of the finding that uraemia prevented the decrease of AlT but not of BATase activity in response to a low-protein diet are discussed, with particular reference to reutilization of urea-nitrogen for the synthesis of non-essential and essential amino acids.