ENIntroduction. Analysis of preserved legal texts is an important instrument in studying the past which provides an opportunity to make conclusions on the level of social and legal development of the society. The modern scientists tend to attribute to legal texts of the past the same meaning that the modern laws have. The objective of this study is to consider to what extent the Lithuanian Statute of 1529 and the Moscow Code of Law (Sudebnik) of 1497 were mandatory for judges. Methods. To study the level of social and legal development of the society, investigators traditionally use the approach associated with the study of application of legal provisions in practical life. In such event, the investigator inevitably undertakes the role of a judge as they need to understand the provision of law in order to determine how it is applied. Consequently, one cannot be sure that modern interpretation of the provision is the same as interpretation of judges in the past. This study tries to find out how often the judges refer to the Statute of 1529 and the Sudebnik of 1497 in their judicial decisions. Results. The data for the Grand Duchy of Lithuania are based on the study of the judicial decisions by Vitebsk voivode M.V. Klochko for the period 1533–1540: he took 6 decisions containing accurate citations from the Statute of 1529 and 9 decisions referring to the provisions of the Statue from nearly 200 decisions. The conclusions for the Grand Duchy of Moscow have been made based on the studies conducted: from the preserved 160 documents dating to the first half of the 16th century in the Grand Duchy of Moscow the judges referred to the provisions of the Sudebnik in 31 cases.Conclusions. The Grand Duchy of Lithuania considerably developed enforcement mechanisms in the first half of the 16th century. This allowed establishing a rule in the Statute of 1529 that a custom could be applied in cases where a suitable provision of law (Statute) was absent. By contrast, the provisions of the Sudebnik of 1497 were mandatory for actions of judges, otherwise the provisions of the Sudebnik were advisory in nature. Key words: the Grand Duchy of Lithuania, the Grand Duchy of Moscow, Statute of 1529, Sudebnik of 1497, law development, law, custom. [From the publication]