Relationships between skin temperature (Tsk) and perfusion have been studied to provide a basis for the use of Tsk in the non-invasive assessment of limb circulation in peripheral vascular disease. Raising the ambient temperature (Ta) from 20 to 30 degrees C increased the perfusion of the glabrous skin of the hands and feet without changing that of the skin of the forearm or calf. On a fractional basis the response in the hand and foot was the same. Tsk was higher in the arms than the legs and in the proximal than distal parts of the limbs. A fall in Tsk was often seen when Ta rose from 20 to 25 degrees C and was attributed to counter-current cooling. Subsequently Tsk rose even in regions where there was no increase in skin perfusion. Tsk can only be related to its perfusion in the fingers, palm and toes. Forearm Tsk was related to the perfusion of the digits. This relationship implies a link with the arterial inflow to the limb which determines the size of its thermal core. Heat conduction from the core seemed important for the skin of areas like the forearm and calf where the constant, low perfusion limited the amount of heat which could be transported to it directly by the blood. The importance of conduction was supported by studies, at Ta 20 degrees C, on subjects during calf muscle exercise and on patients with arterio-venous fistulae. Here an increase in the arterial inflow to the limb was associated with a rise in Tsk of the forearm/calf unrelated to the perfusion of its skin.