- Abnormalities in neuroendocrine regulation are widespread in depressive illness. In this article, abnormalities found in five different endocrine systems are evaluated. There has been a wide-spread use of the dexamethasone suppression test in investigation of depressed patients. Use of this test as a diagnostic test for melancholia may be confounded because abnormalities are found in overlapping illnesses such as Alzheimer's disease or anorexia nervosa as well as in a variety of other conditions such as fasting. However, this test has promise in monitoring clinical status in patients who have an abnormal DST. Abnormalities found in the TRH test and in growth hormone regulation are of limited use clinically, but point to underlying biologic abnormalities. Aberrent regulation of prolactin is now well established, but this hormone has been investigated to a limited extent and warrants further investigation. There is currently a good deal of interest in the pineal hormone melatonin. Reduction in the normal nocturnal peak is found in depressed patients and there is an increase in nocturnal melatonin levels found in patients during treatment with desipramine. Bipolar patients are reported to be abnormally sensitive to melatonin suppression by light. This finding points to the abnormality in the photoperiodic regulation of the pineal output in these patients. Further refinement of neuroendocrine approaches to the investigation of depression should be very productive.