Carbonyl reductase from rabbit kidney was rapidly inactivated by diethylpyrocarbonate (DEPC). A similar inactivation was observed in photooxidation of the enzyme by methylene blue. The inactivation by DEPC was time- and concentration-dependent and followed pseudo-first-order kinetics. The results obtained from the inactivation kinetics and the protective effect of 4-acetylpyridine (4-AP) with NADP+ against the inactivation led to the idea that one essential amino acid is located in substrate-binding domain of the enzyme. The treatment of the enzyme with DEPC formed N-carbethoxyhistidine. Judging from the effect of 4-AP with NADP+ on the formation of N-carbethoxyhistidine, it is concluded that one histidine residue is located in substrate-binding domain of the enzyme. Furthermore, whether one essential amino acid inactivated by DEPC corresponds to one histidine residue modified with DEPC is discussed.