3-Cyano-7-hydroxycoumarin is closely related to 3-cyano-7-ethoxycoumarin which is used as a fluorometric substrate and inhibitor for cytochrome P-450 enzymes and cytochrome P-450-dependent mixed function oxidases. 3-Cyano-7-hydroxycoumarin may be useful to study the kinetics and substrate specificity of cytochrome P-450s. 3-Cyano-7-hydroxycoumarin is closely related to 3-cyano-7-ethoxycoumarin which is used as a fluorometric substrate and inhibitor for cytochrome P-450 enzymes and cytochrome P-450-dependent mixed function oxidases. 3-Cyano-7-hydroxycoumarin may be useful to study the kinetics and substrate specificity of cytochrome P-450s.