Dexfenfluramine was approved in the United States for long-term use as an appetite suppressant until it was reported to be associated with valvular heart disease. The valvular changes (myofibroblast proliferation) are histopathologically indistinguishable from those observed in carcinoid disease or after long-term exposure to 5-hydroxytryptamine (5-HT)₂-preferring ergot drugs (ergotamine, methysergide). 5-HT₂ receptor stimulation is known to cause fibroblast mitogenesis, which could contribute to this lesion. To elucidate the mechanism of “fen-phen”-associated valvular lesions, we examined the interaction of fenfluramine and its metabolite norfenfluramine with 5-HT₂ receptor subtypes and examined the expression of these receptors in human and porcine heart valves. Fenfluramine binds weakly to 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors. In contrast, norfenfluramine exhibited high affinity for 5-HT_2B and 5-HT_2C receptors and more moderate affinity for 5-HT_2A receptors. In cells expressing recombinant 5-HT_2B receptors, norfenfluramine potently stimulated the hydrolysis of inositol phosphates, increased intracellular Ca²⁺, and activated the mitogen-activated protein kinase cascade, the latter of which has been linked to mitogenic actions of the 5-HT_2B receptor. The level of 5-HT_2B and 5-HT_2A receptor transcripts in heart valves was at least 300-fold higher than the levels of 5-HT_2C receptor transcript, which were barely detectable. We propose that preferential stimulation of valvular 5-HT_2B receptors by norfenfluramine, ergot drugs, or 5-HT released from carcinoid tumors (with or without accompanying 5-HT_2A receptor activation) may contribute to valvular fibroplasia in humans.