We induced heat stress in urethane-anesthetized rats (the animals were exposed to an ambient temperature at 42 degrees C), and monitored their colon temperature, mean arterial pressure and local cerebral blood flow. Rats 0, 20, 40 or 80 min after heat stress were sacrificed for determination of c-fos mRNA and protein expression in the paraventricular nucleus (PVN), supraoptic nucleus (SON) and preoptic nucleus (PON). The heatstroke, which appears as profound decreases in both mean arterial pressure and local cerebral blood flow and increases in colon temperature, is produced 80 min after heat stress. We show the c-fos mRNA and protein is strongly induced in all these nuclei of rat hypothalamus after the onset of heatstroke. We conclude that c-fos expression in the hypothalamus during rat heatstroke is associated with hyperthermia, arterial hypotension and cerebral ischemia.