This work focuses on the development of expert computer system which includes elements of artificial intelligence. The system is intended for decision making in the process of medical treatment by choosing medications or combination of medications and treatment tactics. Emphasis of the work is on the creation of knowledge base from existing knowledge in this domain and its computer implementation to provide a hierarchical decision-making process. Knowledge to be included in the knowledge base is taken from guidlines for managment of arterial hypertension published by the European Society of Hypertension (ESH) and by the European Society of Cardiology (ESC) where the newest knowledge about specific medical problems is summarized. Decision making process in the system has a hierarchical structure where following parts can be distinguished: choice of treatment strategy, selection of nine hypotensive medications group or combination of medications, choice of treatment tactics and determination of mutual compatibility of combinations of medications. The decision-making procedure is realized in agreement with structure of decision trees that are constructed using the production rules. The every tree have three levels: level of indications, level of contraindications and level of decisions. The trees start with indication that motivate to choose any of the nine hypotensive medications group and end with two kinds of decisions. The terminal decision affirms the usefulness of selected medications group. The intermediate decision indicates that medications group is useless because of contraindications. Decision-making procedure must be continuing according to other tree branch. Corresponding to the hierarchical structure of knowledge base and decision-making process also expert computer system is designed with a hierarchical structure.