There were pronounced interindividual differences in the plasma protein binding of 1-anilino-8-naphthalenesulfonate (ANS) and phenylbutazone in rats. A highly statistically significant positive correlation was observed between the free fraction of ANS and phenylbutazone (r=0.896, p<0.001), and the possibility of the application of ANS to a clinical test was discussed. Kinetic analysis of ANS binding to individual rat plasma revealed the presence of endogenous competitive inhibitors. Free fatty acids (FFA) were suggested for one of the endogenous inhibitors, since the free fraction of ANS and the FFA concentration were related with a correlation coefficient of 0.609 (p<0.001).