Tyramine and octopamine were identified by means of mass fragmentography in human serum. The peaks identical to the retention time of tyramine and octopamine were found on gaschromatogram obtained from serum sample. Two fragments, M/e 316 and M/e 303 in mass spectrum of tyramine were used in tyramine identification, and M/e 528 and M/e 515 were used in octopamine identification. As the experimental results, the ratio M/e 303 : M/e 316 of the sample was identical to that of authentic tyramine, and ratio M/e 515 : M/e 528 was identical to that of authentic octopamine. These results suggest that tyramine and octopamine could exist in human serum.