The deamination of methylamine (MA) by amine oxidase enzymes has been studied and compared with that of benzylamine (BZ) in homogenates of rat aorta and human umbilical artery by means of a radiochemical assay to estimate the radiolabelled deaminated metabolites produced, and also a spectrophotometric assay to measure H2O2 formation during the metabolism of these substrates. The effects of various inhibitors used in these assays suggest that a semicarbazide-sensitive amine oxidase (SSAO) is predominantly if not wholly responsible for the deamination of both MA and BZ in these tissues. MA was found to have a relatively higher apparent Km (102 microM in aorta; 779 microM in umbilical artery) than BZ (6.8 microM in aorta; 207 microM in umbilical artery) for metabolism by SSAO in these tissues. However, these large differences between species in the apparent Km values for each amine indicate that the biochemical properties of SSAO in human and rat vasculature are not identical. SSAO in human umbilical artery was particularly active towards MA, with a Vmax which was approximately 70% greater than that for BZ as substrate, whereas in rat aorta the Vmax for MA was around 60% of that for BZ. MA is known to occur endogenously in man and other species, and the possibility that it may be a physiological substrate in vivo for SSAO is discussed.