The in vitro effect of metalaxyl on monoamine oxidase (MAO) activity in rat heart was studied. Metalaxyl decreased MAO activity in a dose‐dependent manner. The inhibitory concentration (IC50) for metalaxyl was found to be 19 μmol. Substrate‐dependent kinetic studies demonstrated noncompetitive inhibition as evidenced by decreased velocity of enzyme activity (V_max) without significant change in enzyme‐substrate affinity (K_m)‐ The inhibition of MAO activity by metalaxyl suggests interference with amine metabolism.