Our previous study demonstrated that dietary taurine or glycine supplementation significantly lowered plasma and hepatic cholesterol and triglyceride concentrations in rats fed a cholesterol-free diet. In the present study, the effect of long term dietary taurine or glycine supplementation, for the purpose of preventing and/or treating of hyperlipidemia and other known biological functions, on plasma and hepatic free amino acid concentrations and profiles were evaluated in rats. Three groups of male rats(110-130g) were fed a control diet(CD), taurine-supplemented diets(TSD ; CD+ 1.5% taurine) or glycine-supplemented diet(GSD ; CD+1.5% glycine) for 5 weeks. Plasma and hepatic free amino acid concentrations were determined by an automated amino acid analyzer based on ion-exchange chormatography. The feeding of TSD for 5 weeks yielded a 444% higher plasma taurine concentration , and the feeding GSD for the same period resulted in a 143% higher plasma glycine level in rats compared to those fed DB. Hepatic taurine concentration was significantly higher in rats fed TSD(145% increase) compared to the control rats. However, hepatic glycine concentration was not influenced by dietary glycine supplementation , which implies that the massive dose of glycine entering the body was more rapidly metabolized or excreted than taurein. Dietary taurine or glycine supplementation resulted in similar changes in plasma free amino acid concentrations, except in levels of taurine and glycine. Plasma levels of histidine, lysine, phenylalanine , alanine, proline, hydroxypoline, $alpha$-aminogutyric acid, cystathionine and ethanolamine were significantly higher in rats fed TSD or GSD than those fed GD. Glycine supplementation did not change hepatic free amino acid concentrations as compared to CD. Concentrations of most hepatic free amino acids were not influenced by dietary taurine supplementation with the exception of significantly higher levels of asparate and tyrosine(56-63% increase) and lower levels of histidine and glutamate(33-34% decrease) compared to the control rats. These results suggest long-term dietary taurine or glycine supplementation resulted in increases in most plasma free amino acid levels, but did not cause a characteristic change in plasma aminogram pattern compared to rats fed CD.