Tail skin blood flow (TBFu) was directly measured in anesthetized and passively heated rats by ultrasonic Doppler flowmetry during heating, and the values were compared to those (TBFp) simultaneously measured by venous-occlusion plethysmography. TBFp was estimated from the values per unit tissue multiplied by the tail volume, the shape of which was assumed to be a cone. TBFp was highly correlated with TBFu, with a regression equation of TBFp = 0.7TBFu+0.1 (r2 = 0. 94, p<0.001). Although TBFp was slightly lower than TBFu, the equation is useful to estimate the absolute values of tail skin blood flow from the values of plethysmography in awake rats.