The concept regarding the pathogenesis of pyrexia is almost exclusively concentrated on humoral mediatory mechanism. As per these views, exogenous pyrogens first persuade the formation of pyrogenic cytokines and these endogenous pyrogens order to increase the synthesis of prostaglandin E in different structures of the central nervous system (CNS). The subsequent alterations of CNS functions are reflected in the modifications of peripheral thermoregulatory effector functions, which hoist body temperature during the development of fever and maintain it during the whole fever course. Brewer's yeast-induced pyrexia model has been furnished in the chapter.