All modern Karelian dialects to a greater or lesser extent have retained the Proto-Karelian phonological phenomenon of consonant interchange. However the Lude dialect has completely lost its qualitative form of alternation, and Livvi has lost the alternation of the plosive consonants which are located in the position after voiceless consonants, which is one of the main characteristic features of the Karelian Proper dialect. Apparently, this phenomenon is explained by the phonological representation of the degree of conservation of the Vepsian substrate in southern Karelian dialects, as alternation of consonants is not typical of modern Versian, but if it had been present before (which is questioned), then, obviously, it had already been lost at the moment of the Karelian – Vepsian contact. The paper deals with specific instances of alternation in Lude and Livvi due to Vepsian impact.