Abstract Semicarbazide-sensitive amine oxidase (SSAO) is present in plasma, as well as in other tissues. Previous studies indicated that SSAO is of important physiological and pathophysiological functions. HPLC methods were developed for the assay of SSAO in plasma, and for the determination of plasma methylamine, an SSAO's endogenous substrate. Benzylamine was used as artificial substrate for the enzyme activity assay of SSAO. A 0.2-ml aliquot of plasma was incubated with benzylamine at 37 °C for 30 min. Benzaldehyde, the enzymatic reaction product, was derivatized with 2,4-dinitrophenylhydrazine (DNPH), and analyzed with HPLC and UV detection. SSAO enzyme activity is defined as benzaldehyde (nmol) formed per ml plasma per hour. Recoveries of benzaldehyde spiked to plasma were between 63.5 and 68.2% with relative standard deviation less than 3%. To determine methylamine in plasma, a 0.1-ml aliquot of plasma was deproteinized by trichloroacetic acid and centrifugation. The supernatant was derivatized with dansyl chloride and analyzed by HPLC with fluorescence detection. Recoveries of spiked methylamine at ppb (ng/ml) level were between 93.7 and 97.6% with relative standard deviation less than 2.5%.