CIRCSIM-Tutor is a computer tutor designed to carry out a natural language dialogue with a medical student. Its domain is the baroreceptor reflex, the part of the cardiovascular system that is responsible for maintaining a constant blood pressure. CIRCSIM-Tutor's interaction with students is modeled after the tutoring behavior of two experienced human tutors. The effectiveness of CIRCSIM-Tutor has been tested by 50 first-year medical students. Using a pre-test/post-test paradigm we have demonstrated that significant learning occurs during a 1 hr interaction with the program. Students were also surveyed and indicated considerable satisfaction with the program. (Contains 2 tables and 3 figures.)