Highly unsaturated fatty acids are essential components of cellular membranes of vertebrates and can modulate physiological processes, including membrane transport, receptor function and enzymatic activities. In gilthead sea bream, dietary deficiencies of essential fatty acids of marine fish raise the basal cortisol levels and alter the pattern of cortisol release after stress. The aim of the present study was to clarify the effect of different essential fatty acids on adrenocorticotropic hormone (ACTH)-induced cortisol production and release in fish, through in vitro studies of sea bream interrenal cells maintained in superfusion and incubated with different types of fatty acids and eicosanoid production inhibitors. Results showed the first evidence of the effect of certain fatty acids on cortisol production by ACTH-stimulated interrenal cells in fish. Both arachidonic acid (ARA) and particularly eicosapentaenoic acid (EPA) promoted cortisol production in sea bream interrenal cells. Moreover, incubation with indometacin (INDO) reduced the increased cortisol production induced by EPA and ARA, suggesting mediation by their cyclooxygenase-derived products. Docosahexaenoic acid stimulated cortisol production to a lesser extent than that caused by EPA or ARA, but the inhibitory effect of INDO was not as marked as it was for the other fatty acids. In contrast, supplementation with dihomogammalinoleic acid reduced cortisol production, denoting the inhibitor effect of this fatty acid in cortisol secretion. © 2006 Society for Endocrinology.