Rapid eye movement (REM) sleep deprivation increases monoaminergic (noradrenergic and serotonergic) turnover and their metabolites in whole brain of rats. This increase in metabolites may indicate increased activity of the enzymes responsible for the inactivation of monoamines. To test this hypothesis, we assayed the activity of monoamineoxidases (MAOs) A and B in hippocampus, hypothalamus, brainstem and its divisions pons and medulla oblongata in rats deprived of REM sleep for 96 h. REM sleep deprivation was carried out by the flower-pot technique. A control group remained in their home cages. MAO A was assayed by using [14C]-5-hydroxytryptamine as the substrate (50 microM final concentration) and MAO B by using [14C]-beta-phenylethylamine (2 microM final concentration). The enzymes were assayed in the mitochondrial fraction.The results obtained showed that a significant decrease in the activity of MAO A was obtained in the brainstem and an increase in medulla oblongata and no statistical differences in the activity of MAO B in brainstem, pons and medulla oblongata and MAO A in pons; there were also no differences in the activities of both MAO A and B in hippocampus and hypothalamus. Although our results confirmed previous data regarding changes in MAO A activity in brainstem and medulla oblongata, they did not confirm our hypothesis that the increase in monamine turnover and metabolites in the brain would be the result of increased MAO activity.