Vasodilatation in response to heat, the role of tail vasodilatation in thermolysis and the ability of the animals to use free water for evaporative cooling were studied in control and capsaicin-desensitized rats. 1) While vasodilatation in the tail began in the controls as soon as their temperatures reached 38.5-39 degrees C, in most of the desensitized rats the onset of this reaction was above a body temperature of 40 degrees C. 2) When exposed to 32 degrees C, desalivated capsaicin-treated rats showed a significant hyperthermia as compared to the controls. If the tails were amputated too, the temperatures of the controls rose to near those of the desensitized animals. 3) In the case of prolonged heat exposure or at a higher ambient temperature (38 degrees C), the desalivated tail-amputated controls still remained more tolerant to heat than the desensitized rats. 4) Even at an ambient temperature of 38 degrees C, desalivated controls without tails exhibited excellent thermoregulation when a water bath was present; they used the free water for evaporation. Capsaicin-treated rats neglected the water bath, and their temperatures rose continuously. 5) The results suggests that capsaicin treatment impaired reflex vasodilatation to heat and the behaviour of using free water for heat dissipation.