[3H]Imipramine and [3H]paroxetine label with high affinity a site associated with the serotonin transporter in brain and platelets. The maximum binding capacity (Bmax) of [3H]imipramine in platelets is reduced in untreated depressed patients, and it may represent a useful biological marker in depression. The existence of an endogenous ligand acting on the [3H]imipramine-recognition site to modulate the serotonin transporter has been proposed by several laboratories. 5-Methoxytryptoline inhibits [3H]imipramine binding and [3H]serotonin uptake in the nanomolar range. This compound has been reported to occur in the pineal gland, but probably only in trace amounts. While the physiological relevance of 5-methoxytryptoline or a close analogue remains an open question, the possibility exists that the 'endocoid' for the [3H]imipramine-recognition site plays a role in the pathogenesis of depression.