Written sources have a significant meaning in the study of medieval Moldavia. Numerous amounts of material of the Acts of the 14th-17th centuries was written in the West Russian (South Russian) language. During the 16th-17th centuries the Moldavian language began to slowly be introduced into the affairs of the state. During this period documents appeared, the beginning and end of which were written in Slavonic while the rest was in Cyrillic Moldavian. The structure, linguistic peculiarities, lexicon, titles of the ruling prince and feudal high-ranking officials, information about toponyms, anthroponyms, etc. contained in the material of the Acts present an interest into further study. Rusin influence is noticed in all of the Moldavian diplomacy of the 14th-17th centuries: in various written documents (grievances, grants, confirmations, immunity, legal, protection) and other administrative documents. It is evident in several other customary records of the Valach community which are equivalent with norms of the "Russian Truth" (for example "to track") and in the naming of the heads of the community. At the head of the village community in the 14th-15th centuries a representative of the local nobility was called prince, vatamanor zhude. Urban and village settlements had Slavic names. In medieval Moldavian diplomacy, Slavic names and surnames including those of the feudal lords are seen as a constant occurrence. Of the toponyms mentioned in the documents of the 14th-15th centuries 12.5% are with the Rusin suffix -ovtsy (-evtsy), -intsy (-entsy). During this period, 24.5% are Slavic toponyms. It is also true that to a lesser degree there are also toponyms and hydronyms with the root rus.