Plasma levels of serotonin are elevated in primary pulmonary hypertension (PPH) even after bilateral lung transplantation, suggesting a possible etiologic role. Serotonin is released primarily from the small intestine. Anorectic agents, such as dexfenfluramine, that can cause pulmonary hypertension, are known to inhibit potassium channels in vascular smooth muscle cells. We examined the hypothesis that dexfenfluramine may stimulate release of serotonin from the ileum by inhibition of K⁺ channels. In an isolated loop of rat ileum perfused with a physiologic salt solution, the administration of dexfenfluramine, its major metabolite d-norfenfluramine, the potassium channel blocker 4-aminopyridine (5 mM), and caffeine (30 mM) increased serotonin levels in the venous effluent. Potassium chloride (60 mM) tended to increase serotonin levels. In genetically susceptible individuals, dexfenfluramine may induce pulmonary hypertension by increasing cytosolic calcium in enterochromaffin cells of the small intestine, thus releasing serotonin and causing vasoconstriction. This work indicates that dexfenfluramine and its major metabolite, d-norfenfluramine, can increase serotonin release from the small intestine.