Information on avian fever is limited and in particular very little is known about the mediators and modulators of the febrile response in birds. Therefore in this study the possible mediatory roles of nitric oxide (NO) and prostaglandins (PGs), together with a potential modulatory role for adrenocortical hormones in the generation of fever was investigated in conscious Pekin ducks. Their body temperatures were continuously measured by abdominally implanted temperature sensitive data-loggers. The febrile response induced by im injection of lipopolysaccharide (LPS) at a dose of 100 µg.kg^-1 was compared with and without inhibition of NO production by N-nitro-L-arginine methyl ester (L-NAME), inhibition of PG synthesis (by diclofenac) and elevation of circulating concentrations of dexamethasone and corticosterone (by exogenous administration). LPS administration induced a marked, monophasic fever with a rise in temperature of more than 1°C after 3-4 hr. In the presence of L-NAME, diclofenac and adrenocorticoids at doses which had no effect upon normal body temperature in afebrile ducks, there was a significant inhibition of the LPSinduced fever. In addition, during the febrile response the blood concentration of corticosterone was significantly elevated (from a basal level of 73.6 ± 9.8 ng.ml^-1 to a peak level of 132.6 ± 16.5 ng.ml^-1). The results strongly suggest that the synthesis of both NO and PGs is a vital step in the generation of fever in birds and that the magnitude of the response is subject to modulation by adrenocorticoids.