ENThe author presents the similarities of the Lithuanian language to Polish as well as to other Slavic languages. There are a number of common words: Lithuanian mama, Polish mama, Russian mama, Lithuanian nosis, Polish nos, Russian nos, etc. All these languages have the identical cases in the declension of nouns, moreover, some cases have the same endings; for example: (the masculine dative case of plural) Lithuanian stogams, Polish dachom, Russian krišam. The languages have two grammatical numbers: plural and singular and two genders: masculine and feminine. Slavic languages preserved neuter gender which does not exist in Lithuanian. The author provides a number of examples to prove the archaic character of the Lithuanian language, for example Polish miód and Russian miod are related to Lithuanian medus, however it is impossible to establish the etymology using data from Slavic languages. Lithuanian word medus is closely related to the word medis, showing the place where bees stocked up honey. Russian word medviedj (a bear) - means an animal which knows where honey is (wiedatj- to know). The author presents the Prussian and Yotvingian tribes as well as the etymology of their place names, such as Skalva, Nadruva, Semba, Notanga, Barta, Varmė, Sasna, Pagudė (Polish Pogezania), Pamedė (Polish Pomezania), Galinda, Liubavas territory, Kulmas territory, Dainava, Palenkė (Polish Podlasie) and Sūduva. The author polemicizes with the opinion that most of Prussians and Yotvigians became Lithuanianised. He thinks that this was the natural process of the unification of the Baltic tribes and cannot be compared to Germanization or Polonization.