The ontogenetic development of monooxygenase activities in fetal livers of rats and inducing effects of PCB on them were investigated. The following results were obtained. 1. The activities of monooxygenase systems in fetal livers of rats were detected on Days 17 to 22 of gestation, associated with NADPH-cytochrome c reductase activity and cytochrome P-450 content. All activities and content increased during the development. 2. These activities and content on Days 17 to 22 of gestation were induced by PCB given orally to pregnant rats. 3. The inducibilities of hexobarbital- and aminopyrine-metabolizing activities were very different from that of the benzo[α]pyrene-metabolizing activity. The results would indicate the existence of the different monooxygenase systems in the fetal liver.
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