Monoamine oxidase (MAO) is an outer mitochondrial membrane bound enzyme responsible for the oxidative deamination of a wide variety of biogenic and xenobiotic amines. Altered MAO activity has been reported in a number of medical and psychiatric disorders. Although the tissue distribution of these enzymes has been extensively studied, the localization of MAO proteins and mRNAs at the cellular level in peripheral tissues, including placenta, has not been fully established. In addition the status of placental MAO in pre-eclampsia - a severe disorder of pregnancy has not been fully studied, despite evidence of increased levels of circulating serotonin. This study investigated (a) the distribution of monoamine oxidase (MAO)-A and -B mRNAs and proteins in human peripheral tissues, by in situ hybridisation and immunohistochemistry, respectively, and (b) compared MAO activity, MAO protein and MAO specific mRNA in normotensive and preeclamptic individuals. For the latter, quantitative analyses involving ELISAs and RT-PCR were also conducted. MAO-A and -B mRNAs and proteins were detected in all peripheral tissues, but to differing extents, with similar, strong labelling in kidney glomeruli, duodenal crypts and muscularis mucosae and hepatocytes. However the expression of MAO mRNA does not always reflect protein expression. For example moderate labelling of MAO-A and B specific mRNAs was detected in blood vessels whereas the expression of respective MAO proteins was very low. In contrast, cardiomyocytes showed a moderate labelling for both MAO proteins, but a very weak staining for respective mRNAs. In normotensive placenta both MAO-A and -B specific mRNAs were detected, mainly in the syncytiotrophoblastic layers. The immunohistochemical analysis located only MAO-A protein in these layers. No MAO-B protein was detected in this placental layer, despite the presence ofMAO-8 mRNA. Likewise MAO-A protein was located in the cytoplasm ofthe pre-eclamptic placentae, consistent with a mitochondrial location; no MAO-A protein was found in the nucleus. The results indicate that there was no statistical difference in the expression of MAO-A mRNA in relation to GAPDH or B-actin mRNA or in MAO-A protein/mg total protein. However, MAO-A activity/g fresh weight and the catalytic turnover (activity/molecule) were significantly reduced [77% and 59% reductions, respectively (P< 0.05)], in pre-eclamptic placentae. This study has therefore clearly shown that the expression of placental MAO-A specific mRNA and MAO-A protein are not specifically affected in pre-eclampsia, but that the catalytic efficiency of the expressed MAO-A enzyme in pre-eclamptic placentae is greatly reduced.