Two separate, rapid, sensitive and selective high-performance liquid chromatographic (HPLC) assays were developed for the determination of 4-amino-5-ethyl-3-thiophene-carboxylic acid methyl ester (I) and its acid metabolite, 4-amino-5-ethyl-3-thiophene-carboxylic acid (II), in plasma and urine. The analysis of I is performed directly on a hexane extract of plasma or urine (buffered to pH 11) by normal-phase HPLC analysis using a 10-micron silica gel column with an eluting solvent of hexane-ethanol (95:5) and UV detection of the effluent at 254 nm. A methyl analogue, 4-amino-5-methyl-3-thiophenecarboxylic acid methyl ester, was used as the internal standard. The analysis of II is performed on the residue of either a diethyl-ether-washed protein-free filtrate of plasma or a methylene chloride-isopropanol (95:5) extract of urine (buffered to pH 5.3) using a 10-micron alkyl phenyl (reversed-phase) column with an eluting solvent of water-methanol-1 M phosphoric acid, pH 2.5 (70:30:0.05) with UV detection of the effluent at 254 nm. An isopropyl analogue, 4-amino-5-isopropylthiophene-3-carboxylic acid (IV), was used as the internal standard. The assay of compounds I and II were applied to the determination of plasma and urine concentrations of I and II in the dog and in man following oral administration of I X HCl. The data obtained demonstrated the extremely rapid and virtually complete deesterification of I (ester) to II (acid) in both species.