To examine the developmental expression and regulation of P450SCC and P450(17 alpha) in the rat placenta, trophoblast and decidual tissue were removed by dissection from conceptuses obtained from rats on selected days of pregnancy. Total cellular and poly(A)+ RNA and microsomal and mitochondrial fractions were isolated and analyzed for the presence of P450(17) alpha and P450SCC messenger RNA (mRNA) and protein by Northern and Western blot analysis. P450(17) alpha and P450SCC mRNA were detected in the trophoblast but not in the decidual tissue. Western blot studies demonstrated that the immunoreactive P450(17) alpha in the rat placenta is a 79-kilodalton protein, having a slower mobility in sodium dodecyl sulfate-polyacrylamide gel electrophoresis than has been reported for other tissue. Antiserum preabsorbed with pure P450(17) alpha was unable to detect this protein, and immunoprecipitation indicated that it is associated with enzyme activity. Development studies have revealed that the two steroidogenic enzymes are differentially expressed during the progression of pregnancy. Whereas P450SCC mRNA and protein increase abruptly between days 10-12 of pregnancy, decline thereafter, and remain low, those of P450(17) alpha increase slowly and progressively, peaking on day 18 and declining just before parturition. It is the changes in P450(17) alpha and not that of P450SCC which appear to be intimately linked to the previously reported changes in placental production of androgen. To examine whether P450(17) alpha and/or P450SCC became expressed from midpregnancy because of the rapid decline in LH that occurs at this stage, pregnant rats were treated with low but sustained levels of human CG (hCG) in order to prevent the drop in LH activity. hCG treatment caused a remarkable down regulation in the expression of both P450SCC and P450(17) alpha message and protein. In summary, the results of this investigation have established, for the first time, the presence of messages for both P450(17) alpha and P450SCC in the trophoblast tissue forming the rat placenta. The results have revealed that these two enzymes are differentially expressed during the progression of pregnancy and that the expression of their genes is down-regulated by LH/hCG.