THE distribution of capture cross-sections of slow neutrons reveals a surprising concentration of strongly absorbing elements in the region of the rare earths1. As has been pointed out2, this fact may be explained in the simplest way by the supposition that the mean spacings between the resonance capture levels of slow neutrons have a minimum in this region of the periodic system. A more detailed study of the connexion between the distribution of the capture cross-sections and the variation of resonance level density carried out by me seems to prove definitely the non-monotonic shift of the resonance level spacings with a minimum in the region of the rare earths (mean atomic weight 160).