We have investigated the developmental pattern of expression and activity of 17alpha-hydroxylase/C-17,20-lyase cytochrome P450 (cytochrome P450c17) in the liver, stomach, duodenum, and testis of rats from day 18 of pregnancy to adulthood. In the male liver, the enzyme became detectable at birth (135 pmol/mg protein x min) at a level comparable to that in the testis (188 pmol/mg protein x min). The activity then increased dramatically, reaching a peak at 8 days (691 pmol/mg protein x min), which was more than 4-fold the testicular levels in rats of the same age or in adults. Thereafter it declined steadily, becoming undetectable from puberty onward. The hepatic peak followed a depression in testicular activity (58 pmol/mg protein x min) on day 6. Northern and immunoblot analyses showed a good temporal correlation between enzyme activity and the occurrence of P450c17 messenger RNA (mRNA) and protein. The same patterns of mRNA and protein occurrence were observed in female rat liver, indicating that the hepatic CYP17 expression is not sexually dimorphic. Sequencing confirmed a complete identity in the coding region between hepatic and gonadal mRNAs. Hepatic P450c17 mRNA, however, was 150-200 bases longer than the gonadal counterparts. No significant expression of mRNAs encoding P450scc and P450arom was observed in liver of either sex at any age. In stomach and duodenum, enzyme activity was much lower (maxima at 25 and 14 pmol/mg protein x min, respectively) than that in liver, but persisted from the time of weaning onward. It is suggested that the hepatic peak in P450c17 activity may serve to convert circulating progestogens into androgens for gonadal aromatization during Sertoli and granulosa cell proliferation.