ABSTRACTPulse‐chase labeling experiments with3H‐histidine and3H‐arginine were performed in hairless mice to investigate the origin of free amino acids in the stratum corneum. Time‐course labeling of epidermal proteins was examined by sodium dodecyl sulfate‐polyacryl amide gel electrophoresis (SDS‐PAGE). The radioactivity was also measured in the following three epidermal fractions; 0.1 N HClO4soluble‐ethanol soluble fraction (Fr. I), 0.1 N HClO4soluble‐ethanol insoluble fraction (Fr. II, histidine‐rich protein fraction) and 0.1 N HClO4insoluble‐8M urea soluble fraction (Fr. III).Labeled amino acids were rapidly incorporated into Fr. III, especially into a particular protein which showed little mobility on SDS‐PAGE. As the radioactivity of this protein decreased, an increase in radioactivity was observed in a 32,000 MW histidine‐rich protein, the only major protein in Fr. II. The radioactivity of 32,000 MW protein then fell, accompanied by an increase in radioactivity of Fr. I. The major radioactive substances in Fr. I were identified as3H‐histidine and3H‐urocanic acid after3H‐histidine injection and as3H‐arginine after3H‐arginine injection. The amino acid composition of the histidine‐rich protein was very similar to the composition of free amino acids of the stratum corneum, in which amino acid metabolites were considered as their precursor amino acids.These results suggested that the free amino acids of the stratum corneum might be a final product of the de