In the comparative grammar of Indo-European languages, Baltic languages (Lithuanian, Latvian, Old Prussian) occupy a remarkable position due to the archaisms and richness of their linguistic data. One of the most notable traits is the existence of apophonies, vocalic variations with morphological function. This publication studies the question of the Baltic apophony through a previously hardly examined approach: the connection between radical apophony and grammatical categories (person, case, number, gender, mode and tense).