The presence of 6-hydroxy-5-methoxyindole-2-carboxylic acid (6H5MI-2-C) in normal human urine was demonstrated by mass spectrometry. A quantitative method based on HPLC with fluorometric detection was developed for the determination of 6H5MI-2-C. In normal subjects the mean urinary concentration of 6H5MI-2-C was 38 mumol/mol of creatinine (range 6-76 mumol/mol).