It is a well known fact (Courant and Hilbert, 1962) that any mathematical problem is able to model some physical phenomenon only if this problem is correctly formulated, i.e. its solution exists, is unique, and is stable with respect to input data. In particular, if some mathematical model pretends to describe the temperature field in tissue, it must have such properties because this temperature field really exists, and is uniquely defined by the physical situation. Unfortunately the nonlinear formulation mentioned by Dr. Gray (1979) does not satisfy these demands. Certainly it is possible that a linearised problem does not give sufficiently precise approximation for a real process. The quality of an approximation, however, can be estimated only by comparison with the experimental data. This also relates to the nonlinear model (naturally in those cases when a solution does exist).