The article deals with the reasons behind the appearance of the Slovak Spelling Rules of 1940, the most important changes introduced in the codification of the Slovak literary language, and their significance in the development of the language. The need to develop new spelling rules arose immediately after the publication of the Slovak Spelling Rules of 1931, which were not accepted by the majority of Slovak intelligentsia, who perceived them as the first stage on the way to the absorption of the Slovak language by the Czech one. The new rules were prepared mainly by the leader of the purist movement in Slovakia, H. Bartek, who completed his work in 1939. That version, however, was not approved by the Ministry of Education of the Slovak Republic and was sent for revision. The Rules were published the following year in a modified form and affected the morphology, lexical composition, and orthography of the Slovak literary language. Among the most important changes were the unification of the -l participles in plural forms, a significant reduction of doublet forms by abolishing forms coinciding with the Czech language, the written marking of actually pronounced long vowels in words of foreign origin, the removal from the literary language of a number of lexemes common with Czech, and others. Practically all the changes made contributed to the stabilization of the Slovak literary language norms and were confirmed in the subsequent Slovak Spelling Rules of 1953. Moreover, in 1953 even the proposals of H. Bartek rejected in 1940 were adopted.