1 Administration of chlorpromazine (Cpz), either systemically or centrally, to unanaesthetized rats at an environmental temperature of 23 degrees C caused dose-dependent hypothermia. 2 In order to achieve equivalent hypothermia, intraventricular administration required a total dose of 20 microgram Cpz and and intraperitoneal administration a dose of 9.7 mg/kg body weight. Accordingly, the dose-ratio between intraventricular and intraperitoneal administration was 1 to 110. Cpz apparently exerts its hypothermic effect by acting directly on central nervous structures rather than through peripheral sites. 3 Cpz-induced hypothermia was potentiated by preoptic anterior hypothalamic (POAH) lesions but not by lesions of the ventromedial nucleus (VMN) of the hypothalamus. It was found that Cpz induced hypothermia most readily in rats with large POAH lesions (-10.4 degrees C), less so in rats with spinal lesions (-5.5 degrees C) at least with control rats (-2.9 degrees C).