Prostaglandin E 2 (PGE 2 ) potently activated glycogenolysis and gluconeogenesis in isolated rockfish (Sebastes caurinus) hepatocytes. The average degree of activation for glycogenolysis was 6·4 0·67-fold (mean S.E.M.; n=37), and could be as much as 19-fold. Analysis of doseconcentration relationships between glycogenolytic actions and PGE 2 concentrations yielded an EC 50 around 120 nM in hepatocyte suspensions and 2 nM for hepatocytes immobilized on perifusion columns. For the activation of gluconeogenesis (1·74 0·14-fold; n=10), the EC 50 for suspensions was 60 nM. Intracellular targets for PGE 2 actions are adenylyl cyclase, protein kinase A and glycogen phosphorylase. Concentrations of cAMP increased with increasing concentrations of PGE 2 , and peaked within 2 min of hormone application. In the presence of the phosphodiesterase inhibitor, isobutyl-3-methylxanthine, peak height was increased and peak duration extended. The protein kinase A inhibitor, Rp-cAMPS, counteracted the activation of glycogenolysis by PGE 2 , implying that the adenylyl cyclase/protein kinase A pathway is the most important, if not exclusive, route of message transduction. PGE 2 activated plasma membrane adenylyl cyclase and hepatocyte glycogen phosphorylase in a dose-dependent manner. The effects were specific for PGE 2 ; smaller degrees of activation of glycogenolysis were noted for PGE 1 , 11-deoxy PGE 1 , 19-R-hydroxy-PGE 2 , and prostaglandins of the A, B and F -series. The selective EP 2 -receptor agonist, butaprost, was as effective as PGE 2 , suggesting that rockfish liver contains prostaglandin receptors pharmacologically related to the EP 2 receptors of non-hepatic tissues of mammals. Rockfish hepatocytes quickly degraded added PGE 2 (t Y =17-26 min). A similar ability to degrade PGE 2 has been noted in catfish (Ameiurus nebulosus) hepatocytes, but no glycogenolytic or gluconeogenic actions of the hormone are noted for this species.We conclude that PGE 2 is an important metabolic hormone in fish liver, with cAMP-mediated actions on glycogen and glucose metabolism, and probably other pathways regulated by cAMP and protein kinase A. The constant presence of EP 2 -like receptors is a unique feature of the fish liver, with interesting implications for function and evolution of prostaglandin receptors in vertebrates.