In normal rat hepatocytes in primary culture the level of mRNA encoding the key gluconeogenic enzyme phospho enol pyruvate carboxykinase (PEPCK) is increased by the cyclic AMP analogue, chlorophenylthio cyclic AMP (cpt cAMP), and this response is reversed by insulin. The protein-phosphatase inhibitor okadaic acid diminished the stimulatory effects of cpt cAMP on PEPCK mRNA. Protein kinase A remained fully active in the presence of okadaic acid, therefore, the insulin-mimetic actions of okadaic acid were localised to a site subsequent to initial protein kinase A activation. Insulin produced a decrease in PEPCK mRNA expression which was similar to that of okadaic acid both in extent and mechanism (i.e., lack of change in protein kinase A activation). The effects of okadaic acid on PEPCK mRNA amount were not additive with those of insulin and the effects of insulin were not abolished by okadaic acid. These data suggest that okadaic acid and insulin may interact with the cAMP regulation of the PEPCK gene expression at a common site. The mechanisms by which this may be attained are discussed in relation to what is known about the control of specific protein kinases and protein phosphatases by insulin and okadaic acid and of the importance of protein phosphorylation state to regulation of gene-transcriptional processes.