The binding of [3H]citalopram to the platelet 5-hydroxytryptamine (5-HT) transporter was measured in a group of healthy male drinkers of ayahuasca, a psychoactive sacrament indigenous to Amazonia, and a group healthy male controls. An increased number of binding sites (Bmax)in the platelets of ayahuasca drinkers was found, while the dissociation constant (Ka) remained the same for both groups. If indicative of neuronal 5-HT uptake activity, these results would suggest a decreased concentration of extracellular 5-HT, or a response to increased production and release of 5-HT. Such changes in 5-HT synaptic activity, in this case, should not be misinterpreted as an indication of developing neurological or psychiatric illness. Key words: Beta-Carbolines - Dimethyltryptamine (DMT) - Ethnopharmacology - Harmala alkaloids - Human platelets - 5-Hydroxytryptamine - Psychedelic