The present study was carried out to determine whether interleukin-1 (IL-1) production occurs in the rabbit organum vasculosum laminae terminalis (OVLT) during fever induced by endotoxin. The intravenous (i.v.) injection of endotoxin (4 micrograms/kg) caused significant fever in rabbits. Through the use of in situ hybridization and immunohistochemical techniques, the synthesis of IL-1 was observed in the OVLT during the fever. The present results support the hypothesis that IL-1 is produced in the brain during fever.