A method has been developed for the simultaneous determination of Tranilast, N-(3',4'-dimethoxycinnamoyl)anthranilic acid (N-5'), and metabolites in plasma and urine from humans, dogs and rodents administered N-5'. Total N-5' and metabolite N-3 conjugates were determined in human urine. Detection limits in plasma were 0.2 micrograms/ml for metabolite N-3-S and N-5' and 0.1 micrograms/ml for metabolites N-3 and N-4. In urine, detection limits were 2 micrograms/ml for metabolite N-3-S and N-5' and 1 micrograms/ml for metabolites N-3 and N-4. Metabolite N-4 was not identified in any sample assayed.