The intestinal spasmolytic drug mebeverine is known to undergo fast in vivo enzymatic hydrolysis into mebeverine alcohol and veratric acid. A reversed-phase HPLC method with coulometric detection was developed in order to assay the hitherto unidentified secondary metabolite mebeverine acid. After intake of a single oral dose of 405 mg mebeverine hydrochloride in four healthy human volunteers, peak plasma concentrations of mebeverine acid were found to be 1000-fold higher than those of mebeverine alcohol, i.e. approximately 3 microg/ml versus 3 ng/ml. The appearance of mebeverine acid in plasma (median T(max)=1.25 h) as well as its disappearance (median apparent t(1/2)=1.1 h) were rapid. The urinary excretion of mebeverine acid within the first 4 h after dosing amounted to 67% of the mebeverine dose (median range: 23-107%). Mebeverine acid appears to be a valuable marker of oral exposure to mebeverine.