Various hormonal and monoaminergic systems play determinant roles in the regulation of several cytochromes P450 (P450s) in the liver. Growth hormone (GH), prolactin, and insulin are involved in P450 regulation, and their release is under dopaminergic control. This study focused on the role of D?-dopaminergic systems in the regulation of the major drug-metabolizing P450s, i.e., CYP3A, CYP2C, and CYP2D. Blockade of D?-dopaminergic receptors with either sulpiride (SULP) or 4-(4-chlorophenyl)-1-(1H-indol-3-ylmethyl)piperidin-4-ol (L-741,626) markedly down-regulated CYP3A1/2, CYP2C11, and CYP2D1 expression in rat liver. This suppressive effect appeared to be mediated by the insulin/phosphatidylinositol 3-kinase/Akt/FOXO1 signaling pathway. Furthermore, inactivation of the GH/STAT5b signaling pathway appeared to play a role in D?-dopaminergic receptor-mediated down-regulating effects on these P450s. SULP suppressed plasma GH levels, with subsequently reduced activation of STAT5b, which is the major GH pulse-activated transcription factor and has up-regulating effects on various P450s in hepatic tissue. Levels of prolactin, which exerts down-regulating control on P450s, were increased by SULP, which may contribute to SULP-mediated effects. Finally, it appears that SULP-induced inactivation of the cAMP/protein kinase A/cAMP-response element-binding protein signaling pathway, which is a critical regulator of pregnane X receptor and hepatocyte nuclear factor 1?, and inactivation of the c-Jun N-terminal kinase contribute to SULP-induced down-regulation of the aforementioned P450s. Taken together, the present data provide evidence that drugs acting as D?-dopaminergic receptor antagonists might interfere with several major signaling pathways involved in the regulation of CYP3A, CYP2C, and CYP2D, which are critical enzymes in drug metabolism, thus affecting the effectiveness of the majority of prescribed drugs and the toxicity and carcinogenic potency of a plethora of toxicants and carcinogens.