The phenobarbital induction of five responsive hepatic cytochrome P450 genes is highly strain selective, particularly in female rats (Fischer >> Wistar Furth). We have shown that this strain variation represents a systematic difference in the endocrine-mediated suppression of phenobarbital induction which points to a common signaling process for each of these genes. Immunoblot analysis revealed that the strain-specific differences of phenobarbital responsiveness (10-fold for CYP2B1, CYP2B2, and CYP3A1 in females) are much smaller in male animals and are also greatly diminished by hypophysectomy. Partial depletion of thyroid hormone and growth hormone levels by methimazole treatment was equally as effective as hypophysectomy in elevating phenobarbital-induced levels of CYP2B1, CYP2B2, and CYP3A1 in Wistar Furth rats, while the Fischer strain was unaffected. Ovariectomy suppressed the phenobarbital induction of these genes in the Wistar Furth but not in the Fischer strain, while castration yielded a similar differential suppression in male rats which was reversed by testosterone propionate supplementation. Changes in CYP2B1 protein closely correlated with changes in 7-pentoxyresorufin-O-dealkylation activity, a functional marker for this P450. The strain-selective differences, although smaller, were also observed in the very low basal expression of these P450 genes, while the effects of hypophysectomy, ovariectomy, and castration occurred in a similar manner. However, methimazole was essentially ineffective relative to hypophysectomy in elevating basal expression of these genes. The low concentrations of residual growth hormone and thyroid hormone probably provide a more effective suppression in the basal than in the induced state. We conclude that multiple cytochrome P450 genes share a common phenobarbital induction pathway that, in part, alleviates the suppressive effects of thyroid hormone and growth hormone which are far greater in female Wistar Furth rats. This suppression is opposed by testosterone and to a lesser extent by estradiol. Similar strain differences in the endocrine control of weight gain point to systemic hormonal mechanisms that interface with phenobarbital signaling to control multiple P450 genes.