The metabolism and urinary excretion of aminopyrine was examined in man after an oral administration of the drug solution. The metabolites were identified and measured quantitatively by gas chromatography and gas chromatography-mass spectrometry. For the microanalysis of metabolites, mass fragmentography was applied. A new and significant metabolite, 4-formylaminoantipyrine, was detected in all cases. The individual difference among subjects in their urinary excretion of aminopyrine was also significant.