In order to find an explanation for the WALSH rules on a more theoretical basis the molecules of type AH3 were treated for atoms A of the first period by a simple MO—LCAO method with SLATER-Functions. The dependence of the one-electron energies on the valence angle was found more similar to that obtained by accurate SCF—LCAO calculations than to the WALSH energy curves. But the WALSH rules were generally reproduced very well, except perhaps for the 7 electron case, where in agreement with experiment a more planar structure is preferred. The results are presented as tables and graphs.