Spontaneously hypertensive rats (SHR) are more suitable to heat stroke than are normotensive controls. To determine whether hypertension impairs thermoregulatory reflex cutaneous vasodilation we observed the tail skin vasodilator responses to body heating in male SHR and normotensive Wistar-Kyoto rats (WKY) anesthetized with chloralose and urethan (80 mg/kg and 500 mg/kg ip, respectively). A pulsed-Doppler flow probe was placed on the caudal artery at its origin to monitor tail blood flow (TBF) velocity (the tail skin is the major thermoregulatory organ of the rat), and a catheter was placed in a femoral artery to monitor arterial pressure and heart rate. Internal temperature was measured via a thermistor placed approximately 10 cm down the esophagus into the gastrointestinal tract. During normothermia, TBF was not different between SHR and WKY (0.10 +/- 0.01 vs. 0.15 +/- 0.02 kHz; P > 0.05), whereas tail vascular conductance (TVC) was significantly lower in SHR vs. WKY (0.73 +/- 0.14 vs. 1.88 +/- 0.31 Hz/mmHg; P < 0.05). In response to body heating (tail remained exposed to room temperature), the increases in TBF and TVC were markedly less SHR than in WKY (0.73 +/- 0.14 kHz and 6.03 +/- 1.11 Hz/mmHg vs. 2.61 +/- 0.35 kHz and 22.48 +/- 2.69 Hz/mmHg, respectively). After lumbar sympathectomy, TBF and TVC were not different from the values observed during hyperthermia in both groups. This indicates that the impaired cutaneous vasodilation observed in SHR was not due to failure to withdraw sympathetic vasoconstrictor tone. We conclude that thermoregulatory reflex vasodilation of the cutaneous vasculature is markedly impaired in SHR.