A ratio between two types of monoamine oxidase (MAO) was studied in the mitochondrial fraction of growing human placenta: MAO-A which is predominant in the tissue and the minor MAO-B. Activity of MAO-A with serotonin as a substrate was statistically distinctly lower (by 22%) in placenta within early periods of pregnancy as compared which the mature placenta during the delivery time. In the growing placenta activity of MAO-B with benzylamine as a substrate reached 365% of the MAO-B activity in the mature placenta. The ratio between MAO of the A and B types was altered during pregnancy as shown by inhibitory analysis, carried out using selective inhibitors of the MAO-A and -B types--Lilly 51641 substance and deprenyl as well as with semicarbazide as an inhibitor of benzylamine oxidase. At the early periods of pregnancy the rate of placental MAO-B oxidative deamination of benzylamine was distinctly higher as compared with that during the delivery act. At the same time, content of semicarbazide-sensitive benzylamine oxidase was increased during the delivery period. Possible biological significance of alterations in the amine oxidases ratio in the growing placenta is discussed considering the known property of the MAO-B inhibitor pargyline to cause abortion within early periods of pregnancy.