The influence of photooxidation of bovine neurophysin I in the presence of rose bengal on its amino acid composition and binding ability for oxytoxin and argininevasopressin were studied. Single histidine residue was photooxidized very rapidly without any decrease in the hormones-binding ability. On the other hand, single tyrosine residue was found to be photooxidized almost completely after 240 min of irradiation accompanying a decrease in the hormones-binding ability. No significant changes in other amino acid residues were found even after 240 min of irradiation. Therefore, it is evident that the tyrosine residue has some role in the hormone binding process of bovine neurophysin I.