The effect of pituitary factor on the constitutive and inducible levels of hepatic phenobarbital (PB)-inducible major cytochrome P-450, P-450b and P-450e, in male and female rat livers was studied by immunoblot analyses. Although only trace amounts (approximately 4 pmol/mg protein) of P-450b and P-450e were detected in untreated adult rats, hypophysectomy increased the contents of P-450b and P-450e 58- and 14-fold, respectively, in male rats and 118- and 30-fold, respectively, in female rats. The increases were also observed in treatment with dexamethasone, which suppressed the pituitary function. Treatment with PB increased more effectively the hepatic contents of P-450b and P-450e, but their contents were still 4-fold higher in the male than the female. Treatment of hypophysectomized female rats with PB increased the contents of P-450b and P-450e 4-fold higher than the contents in PB-treated nonhypophysectomized female rats. Consequently, the sex-related difference in their contents was reduced less than 1.4-fold in the hypophysectomized rats treated with PB. Similar results were also obtained from the quantitation of microsomal O-pentylresorufin O-depentylation and testosterone 16 beta-hydroxylation. Either intermittent injection or continuous infusion of human growth hormone, but not of ovine prolactin, into hypophysectomized male and female rats decreased the contents of both cytochromes. These results indicate that growth hormone acts as a repressive factor for the constitutive and inducible levels of P-450b and P-450e in a manner different from the regulation of P-450-male and P-450-female.