Two forms of monoamine oxidase were identified in 15 regions of the brain. These enzymes were ientified by their sensitivity to the drugs clorgyline and N-(1-phenylisopropyl)-N-methyl-N-propionylamine (E 250 or Deprenyl, Hungary) and by their ability to deaminate serotonin and β-phenylethylamine. Enzyme activity was rather uniformly distributed in the brain. There were no statistically significant differences of enzyme activity when compared in three regions of the brain from mentally normal and chronic schizophrenic subjects. There were, however, large variations of enzyme activities within the groups of subjects tested that could have masked true differences of enzyme activity.