By use of monoamine reuptake inhibitors, it is possible to study the intra- and extraneuronal components of monoamine deamination in brain. In both man and rat, the ratios of activities of MAO-A/MAO-B are higher intra- than extraneuronally. As a result of this difference in cellular localisation in the brain, loss of neurones produces a change in the ratios of activities of MAO-A/MAO-B. It is also possible selectively to inhibit the activity of MAO-A within a given neurone population. The α-methyl substituted monoamine amiflamine has been shown to inihibit MAO-A within serotoninergic neurones at lower doses than are required for inhibition in other neurones. The usefulness of this compound in the elucidation of the functions of intra- and extraneuronal MAO are discussed.