The effects of the Chinese herb Chou-Mou-Li, clerodenron fragrans (Ventenaceae) on metabolic, respiratory and vasomotor activities as well as body temperature were assessed in conscious rats at three different ambient temperatures (Ta) of 8, 22 and 30 degrees C. Intraperitoneal administration of Chou-Mou-li produced dose-dependent hypothermia in rats at both 8 and 22 degrees C Ta- At 8 degrees C Ta the hypothermia in response to Chou-Mou-Li was due to decreased metabolism, while at 22 degrees C Ta the hypothermia was due to both decreased metabolism and cutaneous vasodilatation. There was no change in respiratory evaporative heat loss. Furthermore, the hypothermia induced by Chou-Mou-Li was greatly antagonized by pretreatment of animals with p-chlorophenylalanine (a selective depletor of brain serotonin), but not by either atropine sulfate (a selective blocker of cholinergic receptors), regitine (a selective blocker of alpha-adrenergic receptors) or DL-propranolol (a selective blocker of beta-adrenergic receptors). However, at 30 degrees C Ta, systemic administration of Chou-Mou-Li produced no change in rectal temperature or other thermoregulatory responses. On the other hand, direct administration of serotonin into the lateral cerebral ventricle of conscious rats also produced dose-dependent hypothermia at 8 and 22 degrees C Ta. Again, the hypothermia in response to serotonin was due to decreased metabolism at 8 degrees C Ta and was due to both decreased metabolism and cutaneous vasodilatation at 22 degrees C Ta. At 30 degrees C Ta administration of serotonin also produced no change in thermoregulatory functions. The data indicate that Chou-Mou-Li produces hypothermia by increasing sensible heat loss and decreasing metabolic heat production, probably via the release of endogenous serotonin within brain.