The shifts in membrane potential, caused by the injection of some amino acids into a permeation cell, were measured using immobilized serum albumin membranes. The shift in the observed membrane potential apparently was considered to consist of shifts in the membrane potential originating from (a) pH changes in the cell and (b) changes of the fixed charge density in the membrane due to the binding between the amino acids and the serum albumin. The intrinsic shifts in the membrane potential were observed to increase in the following order, D-glucose=methyonine=0 < D-serine < L-tryptophan < L-serine < D-tryptophan < isoleucine < L-alanine < D-alanine. The t3/4 value was found to increase in the following order, D-alanine < serine < L-alanine < tryptophan, where t3/4 indicates the time at which shows 75% of the apparent shifts in the membrane potential. The membrane potential theory provides satisfactory explanations for the apparent shifts and intrinsic shifts caused by the binding between the amino acid and the serum albumin which were obtained experimentally.