The influence of endocrine factors on monoamine oxidase activity (MAO) and on noradrenaline metabolism has been evaluated by measuring platelet MAO activity and plasma levels of 3,4-dihydroxyphenylethylene glycol (DOPEG), the major deaminated metabolite of noradrenaline, as well as serum levels of steroid hormones weekly in 9 young healthy women during one menstrual cycle. A decrease in platelet MAO activity (correlated with high serum estradiol levels) was observed during the ovulatory period. In contrast, plasma free or sulfoconjugated DOPEG remained unchanged throughout the menstrual cycle. These results indicate that the hormonal status should be taken into consideration in studies dealing with platelet MAO activity in depressed women.