We have found a dog family in which there were five cases of increased red cell glutathione corresponding to four to five times the normal concentration without any clinical signs. In the present study, we mainly examined the concentrations of free amino acids in the erythrocytes, plasma, and urine of two of the dogs, and we demonstrated that the concentrations of glutamate, aspartate, and glutamine in their erythrocytes increases to 92, 63, and 13 times the mean value in the normal blood, respectively. There were no changes observed in the other amino acids as compared to normal, although the glycine and histidine in the erythrocyte showed slight increases. The concentrations of amino acids in the plasma and urine of the dogs were almost equal to normal ones. The activities of some of the enzymes involved in the glutathione metabolism in the erythrocytes from the two dogs were all within the normal range. The increased level of glutathione could be explained by the fact that the feedback inhibition of gamma-glutamyl cysteine synthetase by glutathione was released by the high levels of glutamate in the erythrocytes.