Abstract

The importance of drug‐metabolizing enzymes in developing mammals has been recently reevaluated in view of the activities and potential inducibilities of these enzymes. The role of endogenous factors raises the question of whether there is a positive regulation of the expression of drug‐metabolizing enzymes by hormones. In humans, among the different isoenzymes of cytochrome P‐450 described in adult liver, only one is absent in 20‐week‐old fetuses. Epoxide hydrolase and glutathione S‐transferases are active while UDP‐glucuronidation develops postnatally. The consequence of this asynchronous rise of activities is briefly discussed.