A high-performance liquid chromatographic method for the simultaneous determination of 5-aminosalicylic acid (5-ASA), acetyl-5-aminosalicylic acid (Ac-5-ASA) and 2,5-dihydroxybenzoic acid (5-HSA) in human endoscopic intestinal biopsy with electrochemical detection has been developed and validated. A liquid-liquid extraction procedure was used to isolate these drugs from the biological material prior to analysis. The compounds were separated on an Erbasil S reversed-phase column using methanol-critic acid-sodium hydrogenphosphate-heptane-sulfonic acid-disodium ethylenediaminetetraacetate (pH 3) as mobile phase. The method was linear from 1.0 to 300 ng ml-1 for 5-ASA, from 10 to 1000 ng ml-1 for Ac-5ASA and from 0.1 to 10 ng m-1 for 5-HSA. The limit of detection for 5-ASA and for Ac-5-ASA was 1 ng ml-1 and that for 5-HSA was 0.1 ng ml-1. This procedure is suitable for pharmacological and clinical studies of 5-ASA.