The circadian pattern of free cortisol, measured in saliva, was monitored in normal healthy adults (N = 41) for the first half hour immediately after awakening and in a smaller group (N = 8) at timed intervals throughout the day. The endogenous inhibitor of monoamine oxidase A (MAO-AI) was measured in the same saliva samples in order to explore the relationship between circadian activation of the hypothalamic-pituitary-adrenocortical (HPA) axis and MAO-AI. A marked elevation of salivary cortisol was recorded in the first half hour immediately after awakening resulting in a two to three fold increase from the first awakening level. By contrast MAO-AI was highest immediately upon awakening and fell subsequently. Hence the cortisol response to awakening is preceded by heightened MAO-AI. Moreover those subjects who showed more persistently elevated MAO-AI were characterised by a more pronounced cortisol response. An association between MAO-AI and cortisol was also manifest in the diurnal pattern recorded at timed intervals throughout the day. The decline of salivary cortisol from the morning acrophase to the evening nadir was paralleled by MAO-AI. Both patterns of decline were significant (P < 0.01). Taken together with previously reported psychological stress studies these findings suggest a possible relationship between MAO-AI and HPA activity.