The 24 hr profiles of melatonin and cortisol in serum, morning levels of ACTH in plasma, and the dexamethasone suppression test (DST) were investigated in 32 acutely ill patients with a RDC diagnosis of major depressive disorder, 24 patients with a history of longlasting unipolar or bipolar major depressive disorder studied in remission, and 33 healthy subjects. A significant decrease in maximum nocturnal melatonin level (MTmax) was found in the acutely ill depressed patients with abnormal DST compared to both those with normal DSTs and the healthy subjects. The MTmax levels were unaltered when these patients were reinvestigated in remission. A decrease of MTmax was also seen in the group of unipolar and bipolar patients studied in remission. Low nocturnal melatonin is proposed to be a trait marker for major depressive disorder and depressive states with abnormalities in the hypothalamic--pituitary--adrenal (HPA) axis. A significant decrease of ACTH levels at 0800 hr after dexamethasone administration the preceding evening was found in the healthy subjects, the unipolar--bipolar patients in remission, and the acutely ill depressed patients with normal DSTs, but was not found in the acutely ill depressed patients with abnormal DSTs. These findings support the hypothesis that pituitary ACTH regulation is altered in depressed patients with abnormal DST. Morning plasma ACTH before the administration of dexamethasone did not significantly differ between the acutely ill depressed patients with abnormal DSTs, normal DSTs, the patients with unipolar--bipolar disease in remission, or the healthy subjects. Thus, the abnormalities in the HPA axis in depresséd patients are proposed to be due to a hypersecretion of corticotrophin releasing factor (CRF) with a subsequent stimulus-induced pituitary desensitization. A significant decrease of melatonin after dexamethasone was seen at 0800 hr in the unipolar--bipolar patients in remission as well as in the healthy subjects, at 1600 hr and 2200 hr in the acutely ill depressed patients in remission, but not at 0800 hr in the acutely ill depressed patients in relapse. A significant regression was found between MTmax levels and the degree of non-suppression of cortisol at 0800 hr in the DST in the acutely ill depressed patients both in relapse and in remission. Melatonin thus is proposed to be an inhibiting factor for CRF during depression. A trend to a phase-advance of cortisol nadir and melatonin peak was seen in the acutely ill depressed patients with abnormal DST, possibly indicating an involvement of the suprachiasmatic nuclei in the hypothalamus.