Antidepressant agents with properties to inhibit 5-hydroxytryptamine (5-HT, serotonin) uptake in brain tissue and platelets bind with high affinities to neuronal and platelet membranes. [3H]Imipramine, [3H]paroxetine and [3H]citalopram label specific binding sites related to the 5-HT transporter. [3H]Paroxetine and [3H]citalopram appear to be better ligands than [3H]imipramine. The former label a homogenous population of binding sites, whereas the displaceable binding of [3H]imipramine is heterogenous. Recent observations in several laboratories, which have taken the heterogeneity of [3H]imipramine binding into account, indicate that the binding of antidepressants to the 5-HT transporter probably occurs to the same site that binds 5-HT for transport and not to a separate site as previously suggested. Additional bonds to subsites in close vicinity to the 5-HT recognition site may contribute to the binding. No convincing evidence has been presented of the existence of an endogenous ligand other than 5-HT itself that binds to the [3H]imipramine binding site. Recent studies also suggest that repeated treatment of rats with antidepressant agents does not produce any alterations of the binding of [3H]imipramine or [3H]paroxetine to membranes of cerebral cortex. It is also doubtful whether the density of the 5-HT uptake site in platelets measured with these ligands is decreased in affective disorders as first reported.