The article deals with the development of the Polish language and the Polish-Belarusian language relationship in the cities of the Grand Duchy of Lithuania in the period from the sixteenth to the eighteenth centuries. The text concentrated on the example of the city of Grodno situated in the western part of the Duchy. According to the sources, specific features of the Polish language were that it adopted interesting Belarusian lexical loans, Belarusian personal names written in Latin alphabet. The materials collected contain a lot of examples of the Belarusian influence on the Polish phonetic system. The results of the research problem are discussed on the wide historical background: ethnic structureof the city and the cultural contacts of its inhabitants. The author's attempt to describe the problem is based on the numerous valuable sources, preserved and gathered in the Belarusian archives: the council files of old Grodno, the court's records and the Civilian-Military Commission of Grodno district, the inventories of the nobles' and the monasteries' possessionsand other kinds of contemporary sources.