ENA considerable number of the Russian hydronyms of the Oka basin are of Baltic origin. In terms of word formation they coincide exactly with the corresponding Lithuanian hydronyms or at least their bases. Three semantic groups of such hydronyms can be distinguished: (1) hydronyms, containing names of animals; (2) hydronyms, containing plant names; (3) hydronyms, derived from words in some way defining the hydronym or having to do with water.