In order to develop new and potent reversible MAO-B-selective inhibitors, we examined the inhibitory effects of two substrate-analogues, 5-fluoro-alpha-methyltryptamine (5-FMT) and p-chloro-beta-methylphenethylamine (p-CMP) on rat brain MAO-A and -B activity in vitro. Studies of the inhibitory effects of both analogues showed that 5-FMT was a highly MAO-A-selective inhibitor, whereas p-CMP was a highly MAO-B-selective inhibitor. The degree of inhibition by 5-FMT or p-CMP was not enhanced by extending the time of preincubation of the enzyme and either analogue. Dilution studies revealed the inhibition to be reversible in nature, indicating that both analogues are reversible MAO-A-selective (5-FMT) or -B-selective inhibitor (p-CMP) with high selectivity.