The female specific cytochrome P-45015 beta and the male specific cytochrome P-45016 alpha are the major constitutive P-450 forms in rat liver. Their expression is pretranslationally regulated by the sex specific plasma pattern of GH. To measure the mRNA levels of these hemoproteins in studies on their regulation and expression we set up specific solution hybridization assays with in vitro transcribed cRNA probes. An extensive tissue distribution study showed that both P-45015 beta and P-45016 alpha are specifically expressed in the liver. Investigation of the developmental regulation of respective mRNA showed that the onset of sexual differentiation of P-45015 beta and P-45016 alpha expression coincides with the start of puberty. To assess the level(s) of the pretranslational GH regulation of P-45015 beta and P-45016 alpha nuclear run-on transcription analysis was performed. Sex characteristic administration of GH to hypophysectomized rats indicated that GH exerts its dual effects via both transcriptional and postranscriptional mechanisms.