Antipyretic action was found in febrile rats induced by yeast, and body temperature was elevated in hypothermia rats induced by aminopyrine, when 10 g/kg of Guizhi Tang (GZT) was administered per os. The content of prostaglandin E in the hypothalamus of rats was determined with the radioimmunoassay. Administration of GZT in 10 g/kg p.o. could both decrease the PGE2 level of hypothalamic blood in febrile rats, and increase the PGE2 level in hypothermia rats. Antipyretic action of GZT was also displayed when microinjection of PGE2 into the lateral ventricle which produced fever in rats. The evidence was provided that GZT might carry out at least part of the dual-directional regulating action in body temperature through the promotion or inhibition on PGE2 synthesis, release, or metabolism in the thermoregulatory center.