This paper presents an experimental study that examines subjective representation of «morphemic seams» (the junctions between the words’ roots with affixes, including prefixes and flections) of Russian nouns. In the introduction the authors consider the main hypotheses about how the morphological structure of a word can be represented in the architecture of the mental lexicon (a storage of lexical units (words) and their meanings). According to the hypotheses under test, either words are stored there in all possible forms or they are formed using roots and affixes stored separately. There are also mixed models allowing storage of whole-word forms as well as storage of morphemes forming those word forms. Furthermore, it was intended to check whether morphologically related words (derivational morphology) and forms of one word (inflectional morphology) are represented differently in the mental lexicon. The authors used the method of judgment of simultaneity of perceptual events that is new to this area of psycholinguistics and that enabled them to examine the integration of word parts in a single whole. Differences in subjective simultaneity of sequentially presented Russian language word halves were found between the words split at the morphemic junction and across the root. This fact speaks in favor of a special morphological level in the architecture of the mental lexicon. However, the results of the experiment did not show any difference in subjective simultaneity of sequentially presented word halves, split at the morphemic boundary between the root and the flection, and words split at the morphemic boundary between the root and the prefix. Thus, it is impossible to draw a decisive conclusion concerning the differences between derivational and inflectional affixes.