ENThere is a scarcity of research investigating the legal regulations governing water resources in the Grand Duchy of Lithuania in the 16th century. Although the legal system of the Grand Duchy of Lithuania has been analyzed by historians on many occasions, the issues relating to the operation and administration of water resources remain largely unexplored. Beginning in the 1520s, the grand dukes developed a growing interest in the economic effectiveness of the Grand Duchy of Lithuania, in particular the effectiveness of its water resources. The Lithuanian treasury was in dire need of additional revenues to plug budget holes resulting from the Muscovite wars and the financial policies of Queen Bona and King Sigismund II Augustus. The laws endorsed by Polish rulers in connection with their Lithuanian estates testify to effective management practices and honest supervision exercised by the authorities.