The article deals with the analysis of "-oi/-oyu" and "-ei/-eyu" forms functioning in the first part of "Poems in Prose" by I.S. Turgenev. The forms under analyses, though colloquial, function as stylistically neutral ones (unlike many other syllabically variable forms). It means that the choice of the particular form should be determined not stylistically but by some other factors. It has been proved that in most cases the choice of the form is determined by the rhythmic pattern of the text. Consequently, the functioning of these forms demonstrates the aesthetic value of the stress-syllable structure in the text under study as well as the value of its prosodic layer on the whole.