Subcutaneous and perirenal adipose tissue blood flows (ATBF) were measured by the 133Xe washout method before, during and after 4h exercise on a bicycle ergometer. The load corresponded to about 50% of VO2 max (i.e. about 1.7l/min). Subcutaneous and perirenal ATBF increased at an average to 3--400 and 700% of their initial control values respectively. In six of nine measuring sites ATBF remained increased in the hour following work. During work plasma glycerol concentrations increased 8 fold. The core temperature increased 0.9 degree C, skin temperature did not change significantly. During passive elevation of body temperature (core temperature +1.5 degree C; skin temperature +3 degree C) neither subcutaneous ATBF nor plasma glycerol concentrations changed significantly. It is concluded that the increase in subcutaneous ATBF during exercise is not a reaction to increased body temperature.