<p>Cytochrome P450s (CYPs) are enzymes that are found throughout the three domains of life. They function in the metabolism of endogenous and exogenous compounds. CYPs are extensively studied in mammalian systems due to their importance in drug metabolism and are highly expressed in detoxification organs like the liver and intestine. Fish CYP3s are not well understood. CYP3s have diversified in fish and subfamilies A, B, C and D constitute the CYP3 clade in fish. In this study, CYP3C1, CYP3C2, CYP3C3 and CYP3C4 in zebrafish (<em>Danio rerio</em>) and CYP3B4, CYP3B5 and CYP3B6 in medaka (<em>Orzyias latipes</em>) were quantified in hepatic and extrahepatic organs. CYP3C genes were quantified throughout development. All CYP3B and 3C isoforms were detected in all organs except CYP3B4 in male organs and in female brain. CYP3C1-C3 were maternally acquired and expressed in all embryonic stages. Higher expression of some of the isoforms occurred in the liver and intestine of zebrafish and medaka. This is indicative of a possible role in xenobiotic metabolism. Differences in expression between males and females gonad was observed, suggesting a possible role for estrogen in gene regulation. Further research will contribute to characterizing the upstream response elements in order to understand whether estrogens or other compounds are responsible for CYP3 regulation in fish. This knowledge will contribute to understanding the potential function these unique families of CYPs serve for fish.</p> / Master of Science (MSc)