A paper published by Harry H. Pennes in Volume 1 of the Journal of Applied Physiology defined the theoretical basis for a considerable body of analysis performed by many investigators during the ensuing half century. However, during the past decade, the Pennes' model of heat transfer in perfused tissue has been criticized for various reasons, one of which is that his own experimental data seemed to be at variance with the model. More specifically, the shape of the mean temperature-depth relationship measured by Pennes was distinctly different from the shape of the theoretical curve. In this paper, I show that Pennes used an inappropriate procedure to analyze his data and that, when the data are analyzed in a more rigorous manner, they support his theory. Additional support for Pennes' theory is provided by the experimental data of H. Barcroft and O. G. Edholm [J. Physiol. (Lond.) 102: 5-20, 1942 and 104: 366-376, 1946], who had previously studied cooling of the forearm during immersion in water at various temperatures.