Fos expression in the rat diencephalon, brain stem, cerebellum, and spinal cord was examined after warm (33 degrees C) and cold (10 degrees C) ambient exposures. Fos expression was examined with use of immunohistochemical method and the number of Fos-positive neurons in each nucleus was quantitatively analyzed. When rats were exposed to cold ambient, significant number of Fos-positive neurons was found in the lateral septal nucleus (LS), preoptic hypothalamic area (POA), parvocellular paraventricular hypothalamic nucleus (pPVN), lateral preoptic area (LPO), zona incerta (ZI), paraventricular thalamic nucleus (PV), ventromedial hypothalamic nucleus (VMH), subparafascicular thalamic nucleus (SPF), posterior hypothalamic area (PH), supramammillary nucleus (SuM), microcellular tegmental nucleus (MiTg), lateral lemniscus nucleus (LL), lateral dorsal central grey (CGLD), lateral ventral central grey (CGLV), dorsal parabrachial nucleus (DPB), locus coeruleus (LC), dorsal tegmental nucleus (DTg), vestibular nucleus (Ves), nucleus of solitary tract (Sol), spinal cord, and cerebellum. When animals were exposed to warm ambient, the numbers of Fos-positive neurons in the LS, POA, PV, LPO, and SuM were significantly increased to be equal to those of cold ambient. However, after warm ambient exposure the numbers of Fos-positive neurons in the DPB and spinal cord were increased but less than those of cold ambient, and those in the pPVN, VMH, ZI, SPF, PH, CGLD, CGLV, MiTg, LL, LC, DTg, Ves, Sol, and cerebellum were not significantly increased as compared with those of control or cold ambient. Abdominal temperature was not changed during cold ambient exposure, but the temperature was significantly increased during warm ambient exposure.(ABSTRACT TRUNCATED AT 250 WORDS)