A medical expert system for predicting qualitative pharmacodynamic interactions of the cardiovascular system is described. TLCM traces causal paths of drug action through up to three levels of drug action. The three levels which are molecular/receptor level, physiological level and clinical level provide both deep and shallow reasoning in order to overcome the problem of unknowns in medical expert systems. Sparsity of information in pharmacology results from necessity of using non-invasive techniques for monitoring drug effects in the human subject and difficulty in isolating effect from feedback. The qualitative nature of TLCM is another attempt to deal with incomplete information in pharmacology. / Master of Science