Adrenocorticosteroids and serotonergic neurons exert reciprocal regulatory actions, and both are abnormal in depression. We evaluated whether glucocorticoids influence the serotonin transporter in rat platelets and brain by infusing dexamethasone for 26 days, sufficient for replacement of the entire platelet population. Effectiveness was verified by measurement of plasma dexamethasone levels, adrenal atrophy, and growth inhibition. At the end of the infusion, we examined [3H]paroxetine binding to platelet, hippocampal, and cerebrocortical membranes, and [3H]serotonin uptake into platelets and synaptosomes. Dexamethasone slightly reduced platelet [3H]paroxetine binding (12%) and had no effect on binding in brain. Platelet [3H]serotonin uptake was unaffected, but synaptosomal uptake was significantly reduced. In neither platelets nor synaptosomes did dexamethasone alter imipramine's ability to inhibit uptake. Thus, elevated glucocorticoids are not responsible for reduced platelet serotonin transporter expression in depression, nor for resistance to imipramine's effect in platelets in elderly depression; however, reduced synaptosomal [3H]serotonin uptake indicates that glucocorticoids can affect transport efficiency, even when the number of transporter molecules is unaltered.