Eight natural or semisynthesized monoterpenes were examined for their effects on rat brain monoamine oxidase(MAO) using benzylamine as substrate. Thujone and 3-carene were found to have the inhibition effects on rat brain MAO activity; 38% and 95% inhibition at 10^(-3) M respectively. The kinetic study on 3-carene, the most potent inhibitor tested in this study, showed that its MAO inhibition effect was confirmed as uncompetetive type. But (+) pulegon and (-) isopulegon was found to activate MAO slightly.