Abstract Two new methods of introducing non-linear derivative boundary conditions for A.D.I. methods which solve the heat conduction equation in two space variables are suggested. The first method is fast, but less accurate than the second method with respect to the time variable. The second method has the same order of accuracy as the Crank-Nicolson method. The second method is most suited for recalculation of the previous time step with a new set of boundary conditions. The first method allows non-rectangular regions. The second method becomes less efficient if extended to non-rectangular regions.