In three series of 3 dogs each a solution of crystalline amino acids (Vamin) was given by different routes: gastro-intestinally, intraportally of intravenously. The infusion period was 3 hours and the administered amount 3 g per kg body weight. The amino acid concentration in plasma from different sites of the circulation was analysed before, during and after the infusion. In the series of intraportal and of intravenous infusion blood was taken from the hepatic vein and a peripheral vein and in the gastro-intestinal series from the portal vein as well. Tryptophan and tyrosine increased very slightly in all the series and tyrosine also showed a tendency to decrease during the infusion after an initial rise. Glutamic acid and aspartic acid increased quite markedly when given intravenously and intraportally, but there were only very small blood-level changes at gastro-intestinal administration. The intestinal mucosa seems to play an important role in amino acid metabolism. The remaining 14 of the 18 amino acids given showed approximately the same pattern of blood-level changes during and after the infusion.