Using a primary hepatocyte culture in which the mouse Cyp2b10 gene transcription is activated by phenobarbital (PB)-type inducers, we examined the cellular signalling mechanisms associated with PB induction. Low nanomolar concentrations of protein serine/threonine phosphatase inhibitors okadaic acid (OA) and calyculin A blocked the induction of CYP2B10 mRNA. Nuclear run-on assays indicated that OA suppressed Cyp2b10 gene transcription. Pretreatment of the cells with an inhibitor of Ca²⁺/calmodulin-dependent protein kinases {1-[N,O-bis-(5-isoquinolinesulphonyl)-N-methyl-l-tyrosyl]-4-phenylpiperazine (KN-62)} or with a flavonoid, naringin, were completely or partly protective respectively against the OA-mediated suppression of CYP2B10 mRNA. Several other established modulators of protein kinase activities did not greatly affect the induction of CYP2B10 mRNA, nor could they prevent the suppressive effect of OA. Our results indicate that specific protein phosphorylation-dephosphorylation is required for the induction of Cyp2b10 gene expression, which is modulated through multiple endogenous and exogenous signals.