Objective: β-Arrestins play a pivotal role in G protein-coupled receptor desensitization. β-Arrestins interfere in G protein receptor interaction, thus leading to desensitization of G protein-mediated receptor signaling. G protein receptor signaling and its desensitization were previously implicated in the pathophysiology of mood disorders and in the mechanism of action of antidepressant and mood-stabilizing treatments. The present study aims at quantitatively evaluating β-arrestin-1 levels in leukocytes of patients with major depression and the effect of antidepressants on β-arrestin-1 levels in rat brain. Method: β-arrestin-1 measurements were carried out in cortical, hippocampal, and striatal brain regions of rats chronically intragastrically treated with either imipramine, desipramine, or fluvoxamine. Similar measurements were conducted in mononuclear leukocytes of 36 untreated patients with major depression and 32 healthy volunteer subjects. β-Arrestin-1 levels were evaluated through immunoblot analyses using monoclonal antibodies to β-arrestin-1. Results: β-Arrestin-1 levels were significantly elevated by all three antidepressants in rat cortex and hippocampus, while in the striatum no alterations could be detected. This process became significant within 10 days and took 2-3 weeks to reach maximal increase. Mononuclear leukocytes of patients with depression showed significantly reduced immunoreactive quantities of β-arrestin-1. The reduction in β-arrestin-1 levels was significantly correlated with the severity of depressive symptoms. Conclusions: The findings in the rat study suggest β-arrestin-1 elevation as a biochemical mechanism for antidepressant-induced receptor down-regulation. The findings in human subjects support the implication of β-arrestin-1 in the pathophysiology of mood disorders. β-Arrestin-1 measurements in patients with depression may potentially serve as a biochemical marker for depression.