The regional blood flow in the calf was determined simultaneously by thermal measurement and by 133Xe clearance technique. Calf blood flow (Ft) by thermal measurement was accounted for by the equation of the form Ft = (CdTd + Ho - Mb)/rho bCD (Ta--Td), where Cd is thermal capacitance of the calf compartment, Td is the change of calf tissue temperature, Ta is arterial blood temperature, Td is calf tissue temperature, Ho is the heat dissipation from the compartment to the environment, Mb is estimated metabolism of the calf tissue and rho bCb is the product of density and specific heat of blood. The healthy men were chosen for the experiments. Total calf blood flow was 2.53 +/- 1.31 ml/(min-100 ml calf), and muscle blood flow was 2.63 +/- 1.69 ml/(min-100 ml muscle) and skin blood flow 7.19 +/- 3.83 ml/(min-100 ml skin) measured by 133Xe clearance. On the basis of the results, an estimate has been made of the proportions of the calf volume which can be ascribed to skin and muscle respectively. Estimated muscle and skin blood flow were correlated with total calf blood flow (r = 0.98).