Post-dexamethasone cortisol and platelet monoamine oxidase (MAO) activity levels were examined in 50 depressed patients. The incidence of non-suppression was significantly greater in patients with high platelet MAO activity than in those with low activity. Similar results were obtained when males and females were analysed separately. The mean 4 p.m. post-dexamethasone cortisol level was significantly higher in those patients who had high MAO activity than in their low MAO counterparts. Moreover, a statistically significant positive correlation was observed between platelet MAO activity and 4 p.m. post-dexamethasone cortisol levels.