Abstract: Based on archival material for the first time introduced into scientific circulation, the author demonstrates that the staff of the apparatus of the Presidium of the Supreme Soviet of the USSR even before the formation of the constitutional commission on April 25, 1962, began preparatory work for writing the draft of the future third Union Constitution. The author succeeded in determining when, who and by what reasons became the initiator of this work, and what specific proposals were prepared by various departments of the apparatus for reforming the structure of the highest state agencies of the USSR, above all, the Supreme Soviet of the USSR and its presidium. In the article, which is of an interdisciplinary nature, the results of the study of this problem by historians and jurists are briefly summarized, and new archival material is introduced into the scientific circulation which allows to fill the gaps identified by the author in Russian historiography. The specific contribution of the author to the study of the topic under consideration is his discussion of the preparatory work that was undertaken for the creation of the third Union Constitution even before the official formation of the Constitutional Commission of the Supreme Soviet of the USSR, conducted by the staff of the apparatus of its presidium. Of particular interest are the initiatives of directorate and employees of the legal department who proposed to review the functions and competencies of the highest state authorities.