This paper describes the design and development of an instructional planner for an intelligent tutoring system for cardiovascular physiology that assists medical students to learn the causal relationships between the parameters of the circulatory system, to understand how a negative feedback system works, and to solve problems involving disturbances to the system. The instructional planner is responsible for deciding what to do at each point during a tutoring session. It integrates opportunistic control with sophisticated instructional planning, combining lesson planning with discourse planning. The lesson planning is organized in three levels: goal generation, determination of planning strategies, and choice of tactics to refine the goal into subgoals. The mixed-initiative discourse planning is implemented using a two-level approach: pedagogical decision making at the upper level and tactical discourse state-based planning at the lower level in its discourse management network. It generates plans dynamically based on the student model, monitors their execution, repairs plans when necessary, and replans when the student asks a question or makes a comment. (Contains 49 references.) (Author)