The effect of various drugs on the release of [(3)H]-serotonin from synaptosomes of reserpine-treated rats was compared with that obtained with synaptosomes of untreated animals. The increase in [(3)H]-serotonin release induced by d-fenfluramine was virtually abolished by reserpine; the effect of d-norfenfluramine, the main metabolite of fenfluramine, was instead enhanced in synaptosomes of reserpine treated animals. [(3)H]-serotonin release induced by l-isomers of fenfluramine or norfenfluramine was increased or not affected, respectively, after reserpine treatment. The effects of other drugs, known to activate serotonin mechanisms such as metachlorophenylpiperazine and quipazine, like d-norfenfluramine, were increased by the reserpine treatment. The present data show that [(3)H]-serotonin can be released by drugs from two pools with different sensitivity to reserpine. The reserpinized synaptosomes could provide useful information on the mechanisms of action of drugs acting on brain serotonin.