Administration of a combination of the anorectic drugs phentermine and fenfluramine has been reported to cause severe heart valve disorder, probably through altering serotonin signaling. An attempt has been made to determine which of the serotonin receptor subtypes (5-hydroxytryptamine receptor, 5-HT) are normally present in the interstitial cells of the valve. We have isolated and cultured interstitial cells from human heart valves; total RNA was extracted and screened by reverse transcription-polymerase chain reaction for the expression of all subtypes of 5-HT 1. The expression of 5-HT 1B and 5-HT 1D has been demonstrated in interstitial cells isolated from the valve. Members of these subgroups are considered to be important as they have been shown to regulate cardiac function and movement, and are also target sites for the anti-migraine drug, sumatriptan.