- Publisher Summary This chapter describes brain eicosanoids and lipopolysaccharide (LPS) fever and presents the data obtained at a laboratory on rodents, including rats of a wide range of ages, and birds. The pyrogen applied traditionally in studies of fever is bacterial endotoxin, obtained from various pathogenic gram-negative bacteria. They express on their surface substracts (endotoxins), which are essential for bacterial growth and survival. The LPS exerts its pyrogenic features by inducing cascade of changes that effect on thermosensitive neurons in hypothalamic thermoregulatory center promoting them to a new level of functional activity. The results of the study presented in the chapter demonstrate that different species and strains of rodents (rats and mice) and birds (chickens) exhibit rather specific fever response. Systemic administration of LPS causes monophasic elevation in T b of chickens, biphasic changes in T b of rats, whereas mice fail to develop hyperthermia and respond by a decreased T b . The LPS-induced alterations in hypothalamic prostanoid synthesis are rather species-specific and differ markedly even between the two strains of mice.