6-Methoxy-1,2,3,4-tetrahydro-β-carboline (6-MeO-THBC), which specifically elevates serotonin in the brain, exerted no significant effects on mouse brain monoamine oxidase, rat brain tryptophan hydroxylase, and rat liver tryptophan pyrrolase. There was a marked activation of rat plasma and liver 5-hydroxytryptophan decarboxylase and a concomitant increase of serotonin levels in the two tissues. 6-MeO-THBC did not alter the endogenous tryptophan levels in rat plasma and brain. A significant facilitation of the uptake of [3-14C]-5-hydroxytryptophan was observed in brains of mice treated with 6-MeO-THBC. The possibility that the increased serotonin was derived from 6-MeO-THBC itself was ruled out.