BANERJEE, K.K., BISHAYEE, A. and CHATTERJEE, M. Effects of Human Placental Extract on Brain Monoamines and Monoamine Oxidase Activity in Rats. Tohoku J. Exp. Med., 1995, 176 (1), 17-24-Human placental extract, an agent clinically used world-wide in a number of physiological anomalies, has been claimed to be effective in children of slow learners. Since the monoaminergic neurotransmitter systems in the brain play an important role in the processes of learning and memory, we examined the effects of human placental extract on the levels of norepinephrine, dopamine and serotonine in rat brain as an attempt to evaluate the possible underlying biochemical mechanism of action of the extract. We also determined the changes of brain monoamine oxidase (MAO) activity following placental extract treatment. The results showed that subchronic (5, 10, 15 or 20) administration of placental extract (2-4ml/kg/day) had the effect of increasing all the monoamines and decreasing the MAO activity which could be the possible mode of action of the extract in slow learners.