Two forms of enzymatic posttranslational modifications of the monooxygenase system are described: modification by phosphatase and modification by protein kinase. Phosphatase treatment of microsomes isolated from phenobarbital-pretreated rabbits and rats caused a marked decrease of monooxygenase activity which was paralleled by a comparable decrease of NADPH-cytochrome P-450 reductase activity while the second essential component of the system, cytochrome P-450, remained unaltered. Thus phosphatase attacks monooxygenase via reductase. Protein kinases showed the opposite preference; while cytochrome P-450 was phosphorylated, NADPH-cytochrome P-450 reductase was not. Thus the kinase affects monooxygenase via cytochrome P-450. The phosphorylation of cytochrome P-450 turned out to be a specific reaction observed only with certain cytochrome P-450 isoenzymes and certain protein kinases.