The intravenous injection of 15 micrograms capsaicin produced an increase in the temperature of tail skin and paw pad and a fall in the colon temperature in conscious rats. These reactions reflect increased heat dissipation. The increase in skin temperature induced by intravenous capsaicin was absent when the function of small diameter primary afferent neurones was impaired by treatment of the rats with capsaicin as neonates. Thus it appears that intravenous capsaicin triggered the thermoregulatory response predominantly by stimulation of peripheral heat receptors. By means of local application of capsaicin to the nerves of the hind leg and by their chronic denervation, by treatment with phenoxybenzamine and guanethidine, evidence was obtained that reflex withdrawal of sympathetic vasoconstrictor tone mediates the heat loss reaction intravenous capsaicin.