The aim of the article is to detect lexical elements making the basis of future colloquial speech in the light of research of the colloquial speech formation in Old Russian. Colloquial speech is a dynamic phenomenon which can be interpreted from the functional (as a subsystem of the standard language) and stylistic (as language means) points of view. Heterogeneity and historical variability of the colloquial speech category are important. The material of the research is formed by seven volumes of the Dictionary of the Russian Vernacular of Muscovite Rus of the I6thl7th Centuries and its card file stored in Saint Petersburg State University. The course of the research assumed finding answers to the following questions: What is the status of colloquial speech in the history of language? What are the criteria for distinguishing colloquial lexicon in the Russian language of the 16th-17th centuries? What lexicographic interpretation should this lexicon have in a historical dictionary? To answer these questions, th