The distribution of the heating potential inside a lossy sphere having the same electrical characteristics as those of brain tissue was investigated in the frequency region of 10 MHz to 12 GHz. The conditions under which a potential hot spot appears inside the sphere and its shape were determined and were displayed in a radius vs. frequency diagram. The results show that hot spots appear inside only for spheres with radii 8 cm > R > 0.1 cm and only in the frequency range of 300 < f < 12,000 MHz. It was found that the heating potential is always non-uniformly distributed, and in cases where it was maximum inside the sphere it was larger by a factor of approximately ten to the values calculated by the plane slab or the average cross section model.