Аннотация:This paper discusses the origin of the Circum-Baltic modal construction with the nominative case-marking on the infinitival object: the same verbs assign the accusative case to their object in finite clauses. Independent infinitival clauses with the NOM-INF pattern has the semantics of alethic (external) modality: either "p is necessary" or "p is possible", e.g. the Old Russian example TOLIKO VODA(NOM) PITI (INF) conveys the meaning 'It is only possible to drink water there'. There are two main approaches towards the origin of this construction in Baltic and Slavic. The first approach is based on the reanalysis hypothesis: the nominative argument presumably originated as the subject of the archaic gerund-type Indo-European construction with the approximate meaning 'This water (NOM) is potable", while this archaic construction was inherited by the Baltic and Slavic languages from Proto-Indo-European. The second approach is based on the grammatical borrowing hypothesis: Baltic and Slavic languages presumably borrowed the construction with the nominative case-marking on the infinitival complements from the substratum Finno-Ugric languages. There are also attempts to reconcile these approaches by a stipulation that later Finno-Ugric influence superimposed on the inherited Indo-European construction. I argue that the grammatical borrowing hypothesis is more reliable than the hypothesis on the inherited Indo-European syntactic pattern since the infinitival syntax in Baltic and Slavic languages is different. Slavic languages have a longer written period, while Baltic languages show a larger variety of infinitival patterns. Moreover, of all Slavic dialects, the NOM-INF construction is characteristic only for the Northwestern and Western Russian dialects, which developed in a contact with the neighboring Baltic dialects. I follow the geographical and genre distribution of the NOM-INF construction in the non-bookish Russian texts from ca. 1100-1500 localized in the Great Novgorod, Pskov, and Polotsk-Smolensk areas and argue that two features in Old Russian syntax, which occasionally show up in the texts from this group-1) the "potable water" gerund-type construction with the secondary agreement and 2) the embedded NOM-INF structures after an overt modal verb or predicative-represent not the relics of the inherited Indo-European syntax, but a possible borrowing from the neighboring Baltic dialects.