In order to study heat dissipation ability of spontaneously hypertensive rats, threshold core temperatures for occurrence of the tail vasodilation were compared among adult male spontaneously hypertensive rats (SHR), stroke-prone SHR (SHRSP) and Wistar-Kyoto rats (WKY). Mean age of month (M.) and mean systolic blood pressure were 8.4±0.5 M; 205±5 mmHg for SHR, 7.2±0.6 M; 240±8 mmHg for SHRSP and 7.6±1.1 M; 140±6 mmHg for WKY. For the measurement, each rat was placed into a small wire-mesh cage for a loose restraint. Rectal temperature, tail skin temperature and air temperature were simultaneously recorded by means of thermistor probes or copper-constantan thermocouples. Air temperature was gradually raised at a constant rate of 0.17℃/min. Stepwise changes in the tail skin temperature were observed during the gradual elavation of the air temperature. The beginning of the tail vasodilation was detected by the 1st increase in tail skin temperature. Mean threshold core temperature for the 1st vasodilation in SHR, SHRSP and WKY were, 38.9±0.1℃, 38.4±0.1℃ and 38.0±0.1℃, respectively. Threshold core temperatures for the 1st vasodilation in SHR and SHRSP were significantly high compared to that in WKY, while the threshold in SHRSP was significantly lower than SHR. Mean values of threshold Ta for the 1st tail vasodilation in SHR, SHRSP and WKY were 30.1±0.5℃, 29.9±0･6℃ and 29.7±0.5℃, respectively. There was no statistical significance among these values. These results are consistent with the fact that the core temperature in adult SHRSP is not always high as SHR, even though blood pressure of SHRSP is higher than SHR. And these facts also suggest that high systemic blood pressure itself may not cause developing high core temperature in these genetically hypertensive rats.