The article deals with the use of the Ruthenian language in papers of the Uniate Church of the Grand Duchy of Lithuania in the 17–18th centuries. The research is based on the analysis of the content of hundreds of original parish registers of Uniate churches that are kept in the National Historical Archives of Belarus. The methods of analysis that permit to define the time and reasons for translation of concrete parish works into the Polish language have been devised by (...) author. The collection of data obtained during the research gives a possibility to distinguish several imaginary regions of intensity of using the Ruthenian language. It also considers lexical and the orthographic features of the language in which parish registers were being kept, use of the Cyrillic and Latin script in writing the Ruthenian text. (shrink)