Hypophysectomy of male adult rats caused a 70% decrease in the hepatic level of mRNA hybridized to two specific oligonucleotide probes for the sequence of coding and 3'-noncoding regions of P450(M-1) (H. Yoshioka et al., (1987) J. Biol. Chem. 262, 1706-1711), which corresponds to P450-male. Treatment of hypophysectomized male and female rats with subcutaneous injection of human growth hormone twice a day for 7 days increased the mRNA to a level similar to that of normal male rats. In contrast, the mRNA was decreased by treatment with continuous infusion. These results correlated well with those on the amounts of P450-male protein, indicating that growth hormone regulates the hepatic level of P450-male protein mainly by acting at the pretranslational step. Treatment of adult male rats with phenobarbital (PB), dexamethasone (Dex), or 3-methylcholanthrene (MC) decreased the content of P450-male protein by 68, 36, and 46%, respectively. The content of P450-male protein was also decreased to 65% in Dex-treated hypophysectomized male rats, but was not changed by treatment of hypophysectomized male rats with PB or MC, suggesting that PB and MC decrease P450-male protein through a pituitary growth hormone-mediated process. However, the level of mRNA hybridizable to the P450-male oligonucleotide probe was not decreased, but rather it increased in PB- or Dex-treated hypophysectomized male rats. A similar inconsistent change in protein and mRNA was also observed in PB-treated normal rats. These results indicate that PB and Dex have an additional effect of increasing the hepatic level of the specific mRNA of P450-male/(M-1) or a closely related form. Noncoordinate changes in the level of P450-male protein and mRNA also suggest that the hepatic level of P450-male protein is regulated by plural mechanisms: pretranslational and translational regulation in which pituitary growth hormone and/or other endocrine factors are involved.