SUMMARYDexamethasone suppression test (DST) were performed in patients with dementia of the Alzheimer type and multiinfarct dementia. Test-retest reliability was found to be high in dementia patients if they were inpatients and environmental factors were kept stable. In this group 14% were non-suppressors. If outpatients were investigated, admitted to hospital for only a few days, the test-retest reliability was low. In these patient groups 39-56% were non-suppressors.Treatment with the serotonergic (5-HT) uptake inhibitor citalopram significantly reduced the postdexamethasone cortisol levels as well at 08.00 am as at 03.00 pm the day after intake of 1 mg dexamethasone (after 10 h and 17 h respectively). Citalopram treatment also caused significant reduction of the cerebrospinal fluid (CSF) levels of 5-hydroxyindolacetic acid (5-HIAA) and 3-methoxy 4-hydroxyphenylglycol (HMPG). A correlation was seen between the citalopram-induced reduction of 5-HIAA levels in CSF and the degree of dexamethasone-induced suppression of cortisol concentrations after the treatment period. Treatment with the 5-HT uptake inhibitor alaproclate, with a regional selectivity principally to the limbic system, had little effect on the postdexamethasone cortisol levels. The results suggest a relation between the abnormal DST in dementia and a disturbed hypothalamic 5-HT function.K t Y w o m s Dementia disorders, dexamethasone suppression test, reproducibility studies, environmental effect, effect of two 5-HT uptake inhibitors.