Summary — Hayek and Bruno Leoni were bound together by a long and profitable friendship. Both of them were members of the Mont Pelerin Society, which had Hayek as founder (and president for several years) and Leoni as secretary (he was also president for few weeks). The unpublished letters that here are introduced witness the intellectual importance of their sodality. Leoni replies to the new constitutional system proposed by Hayek, explaining the reasons of his disagreement; Hayek, in his answer, states the basic lines of the model of a new Rechtsstaat which he is working out. Such letters give us the chance to lay stress on similarities and differences in their respective theories of law.