1. Quantitative relationships between molecular physico-chemical properties of 22 substituted benzoic acids and the extent of excretion of their metabolites in rat urine have been investigated using computational chemistry and multivariate statistics. 2. A data set of 34 theoretically derived physico-chemical descriptors calculated was used to classify the benzoic acids according to their predominant urinary metabolic fate. 3. Quantitative structure-metabolism relationships were obtained by linear regression using combinations of physico-chemical descriptors allowing the prediction of % urinary excretion of glycine (r = 0.73) and glucuronide conjugates (r = 0.82) and % urinary excretion of the parent compound (r = 0.91).