The effects of preganglionic decentralization of (sympathetic trunk resection) or post-ganglionic excision (ganglionectomy) of the superior cervical ganglia on cerebral blood flow and the formation of heat stroke were assessed in rabbits. An intravenous isotope method for external measurement of cerebral circulation time was applied to rabbits for determining cerebral blood flow. Heat stroke was induced by exposing animals to a high ambient temperature of 40 degrees C. The occurrence of loss of sensation, decreased muscle tone and unconsciousness was taken as the onset of heat stroke. The results showed that decentralization of the superior cervical ganglia enhanced the cerebral blood flow, whereas ganglionectomy reduced the cerebral blood flow. In addition, the latency for the onset of heat stroke and the survival time after the heat stroke were greatly prolonged by the former surgical procedure, but shortened by the later one. The data suggest that decentralization of the superior cervical ganglia attenuates formation or development of heat stroke by promoting an increase in cerebral blood flow in rabbits.