Quantitative relationship between chemical structure and oral bioavailability of 188 non-congeneric organic medicinals were studied to construct an expert system for predicting pharmacokinetic properties of organic chemicals. The compounds studied were classified into three groups: non-aromatics, aromatics, and heteroaromatics. Their oral bioavailability data observed in human adults were allotted into three ratings, and the relationships with chemical structure were analyzed using fuzzy adaptive least-squares. Quantitative relationship models formulated for the three structure groups gave significant information about factors influencing bioavailability, and were statistically reliable in both recognition and leave-one-out prediction despite the diversity and complexity of the structures of the compounds investigated.