One of the problems of diachronic reconstuction of Slavic borrowings in Latvian is the problem of the differentiation of loanwords and inherited words in the Latvian language, which were acquired as elements of common Balto-Slavic lexics. The problematic nature of this question is conditioned by three factors. The first factor is related to the fact that many Slavic borrowings in the Latvian language belong to very old periods of linguistic contacts between the Eastern Slavs and ancient Latvian tribes; the process of these contacts is not represented in the written historical sources. The second factor is related to the fact that both the Proto-Baltic and Proto-Slavic languages were historical dialects of the Proto-Indo-European language, and were very close to each other. Finally, the problem of differentiation is complicated because of the fact that in the history of the Latvian language there appeared Slavic loanwords that go back to the same historical root as the inherited words from the Balto-Slavic vocabulary. The reflection of these factors is illustrated in the paper through the example of the lexical pair - Latv. greda 'pile, ridge' - Latv. grida 'floor'.