Piperine is an important compound from black and white pepper which had been reported to have stimulating effect to the central nervous system (CNS). It is very interesting to investigate the possible pharmacological actions of piperine which may be related to this effect. Monoamine oxidases (MAO) are the enzyme system located in the mitochondrial outer membrane of many cell types with an important role in destroying monoamine compounds and controlling the body levels of biogenic amines. Therefore, the study of effects of piperine on MAO activity in experimental animals may provide possible explanation to its reported CNS stimulating effects. In this study, the assay of MAO activity was developed by measuring oxygen consumption rate as the indicator of enzymatic activity. This assay method was used to investigate the effect of piperine on enzymatic activity of MAO toward various monoamine substrates and the results obtained suggest that piperine possesses remarked inhibitory action to rat liver mitochondrial MAO activity. The inhibition was, presumably, not competitive in nature and the inhibitory profile did not show preferential effect on specific type of MAO. The preliminary result from the study on rat brain mitochondrial preparations was not consistent and conclusive interpretation based solely on such data cannot be made. Overall, the experimental results suggest the possible relationship between MAO inhibition and CNS stimulatory effect of piperine.