The pyrogenic interferon inducer polyinosinic:polycytidylic acid (Poly I:C) was shown to activate the rabbit hypothalamo-pituitary-adrenocortical (HPA) axis in vivo. The immunoreactive cortisol response to Poly I:C (2.5 μg/kg) was shown to have a corticotrophin-releasing factor-41 (CRF-41)-dependent component which was abolished by peripheral immunoneutralization using an anti-CRF-41 monoclonal antibody (KCHMB001; 2.5 mg/kg i.v.). Peripheral administration of the arginine vasopressin (AVP) V1 receptor antagonist ([deamino-Pen1, O-Me-Tyr2, Arg8]-vasopressin; 225 nmol/kg i.v.) had no effect on the response of immunoreactive cortisol to Poly I:C, suggesting that AVP was not involved in activation of the PHA axis. Poly I:C increased both body temperature and circulating immunoreactive prostaglandin E2; these responses were abolished by the cyclo-oxygenase inhibitor ketoprofen (3 mg/kg s.c.). The immunoreactive cortisol response to Poly I:C, however, remained after the administration of ketoprofen, indicating a prostaglandin (PG)-independent component. The immunoreactive cortisol levels in control, saline vehicle-treated, animals were reduced by both the CRF-41 receptor antagonist (α-helical CRF (9-41); 6.25 nmol/kg i.v.) and ketoprofen (3 mg/kg s.c.) indicating that this basal state is dependent on both CRF-41 and PGs.