Ontogeny of fever.

The newborn of the sheep and the guinea pig are refractory to bacterial pyrogen. The newborn lamb does not get fever and the newborn guinea pig only gets fever to large doses of intravenous bacterial endotoxin. It would appear that the lack of febrile response to endotoxin occurs beyond the stage of production of endogenous pyrogen and may represent either an immaturity of the brain mechanisms for heat production and heat conservation, which are driven by endogenous pyrogen, or, as evidence is presented, there may be a circulating endogenous antipyretic substance present in the circulation of the newborn lamb and there is evidence that such a substance is present in the circulation of the ewe close to term. Possible relevance of these findings to lack of fever in the infected human neonate is discussed.