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The distribution of catalytic activity of platelet monoamine oxidase (MAO) with both tryptamine and phenylethylamine as substrates was examined in 1,129 Swedish men at age 18 years. A mixture of five components was needed to describe the distribution, even when the original scale was transformed to remove skewness. The proportions of admixture were 2% for the extremely low component with a mean of -2.3 sigma, 29% for moderately low MAO (mean -0.8 sigma), 51% for intermediate MAO (mean 0.0 sigma), 15% for moderately high MAO (mean + 1.3 sigma), and 3% for extremely high MAO (mean + 3.0 sigma). Thus, the upper and lower deciles each contain contributions from two extreme components that differ from a much larger intermediate component with activity near the mean of the general population. This is compatible with a minimum of three alleles at a single major locus or with at least two polymorphic loci. The hypothesis that MAO activity is controlled by two alleles at a single locus was tested and rejected. The demonstration of at least five distinct components to the distribution of MAO warrants further research to characterize the biochemical structure and function of MAO enzyme variants as well as study of the behavioral correlates of the components.