: In eel (Anguilla japonica), exposure to polyaromatic hydrocarbons such as 3-methylcholanthrene leads to induction of two CYP1A enzymes, CYP1A1 and CYP1A6. We studied the time course and tissue specificity of induction of messenger RNAs for CYP1A1 and CYP1A6 in eel by administering 3-methylcholanthrene intraperitoneally. In both cases, the drug induced a rapid increase of mRNAs and biphasic expression. In the liver, mRNA levels of CYP1A1 and CYP1A6 increased 22-fold at 3 hours and 27-fold at 6 hours after the administration, respectively, showing initial peaks in the induction. After the initial inductions, mRNA levels decreased unexpectedly. Following these temporary decreases, the mRNA levels again increased and reached levels that were 35 and 41 times the basal levels at 24 hours after administration, respectively. CYP1A1 and CYP1A6 resembled each other also in the tissue specificity of gene expression; the expression levels were liver >> gill > intestine > kidney. The rapid induction, the biphasic expression, and the tissue-specific expression were common features of gene expression in CYP1A1 and CYP1A6 and may come from common structures of the regulatory regions of the two genes.