One of the main typological peculiarities of the Standard Modern Russian phonetic system (in its codified Moscow variety) is the rhythmical structure of the phonological word: stressed and prestressed syllables are opposed to other unstressed syllables in terms of duration and vowel quality, forming the so called “word’s prosodic nucleus”. Some modern Russian dialects as well as some modern regional variants of Standard Russian have no prosodic nucleus at all. The paper deals with the problem of the historical development of the rhythmical structure of the phonological word focusing on the hypothesis that different types of rhythmical structure are determined by variation in the timing of the tonal accent.