The febrile responses of male Sprague-Dawley rats to a semi-purified endogenous pyrogen (EP) derived from human monocytes are markedly enhanced 3 days after the animals are intravenously injected with a variety of immunoadjuvants. The present study was designed to investigate the site within the body at which these substances act to produce this febrile-enhancing phenomenon. Stainless steel microinjection cannula guide tubes were implanted within the region of the organum vasculosum lamina terminalis (OVLT) of the rats and control febrile dose-response curves to EP were established. Minute quantities of the immunoadjuvants zymosan, lipopolysaccharide endotoxin, and the synthetic adjuvant peptide, muramyl dipeptide, were microinjected into the OVLT region and 3 days later, the febrile responses of the animals were retested. In each case the febrile response elicited by a standard dose of EP was more than doubled, the slope of the fever dose-response curve was tripled, and the dose threshold was lowered by a factor of four to five. These responses are identical with those produced when much larger amounts of these immunoadjuvants are injected intravenously, and, thus, we conclude that the site of action of these substances in enhancing fever in response to EP resides in or near the OVLT region. It is proposed that EP stimulates a type of reticuloendothelial cell residing within the OVLT to release prostaglandin E, which in turn crosses the blood-brain barrier to effect the changes in the thermoregulatory neurons of the preoptic anterior hypothalamic area that result in fever.