This report describes a series of in vitro studies of 5HT2B activity of fenfluramine, MDMA (racemic and both R-(-)- and S-(+)-MDMA) and other drugs, and in vitro studies of human heart cell growth arising after 5HT2B receptor stimulation. A receptor activity profile found that MDMA and fenfluramine act as 5HT2B agonists. (The full receptor profile for MDMA is presented but not discussed beyond 5HT2B activity, and is the first to appear since 1988, when the first assay of pharmacological activity was performed (Battaglia et al. 1988)). A study with recombinant cells with human 5HT2B receptors found 5HT2B agonist activity after racemic MDMA (EC50 = 2000 nM, 5.8 ± 0.1 nM, relative efficacy = 0.32 ± 0.02) and MDA (EC50 = 190 nM, 6.73 ± 0.05 nM, relative efficacy = 0.80 ± 0.02.). Both R-(-)-MDMA and S-(+)-MDMA showed human 5HT2B agonist activity, but S-(+)-MDMA was slightly more potent than R-(-)-MDMA (EC(50) for S-(+)-MDMA = 6000 at 5.2 ± 0.2 nM, efficacy = 0.38 versus EC(50) for R-(-)-MDMA = 900, 6.0 ± 0.2 nM, relative efficacy = 0.27). Sufficient drug levels for 5HT2B stimulation were found to be similar to human plasma levels after 150 mg MDMA (De la Torre et al. 2000). R-(-)-MDA and S-(+)-MDA did not exhibit any differences in potency as human 5HT2B agonists. Fenfluramine, norfenfluramine, dihydroxyergotamine, pergolide and methysergide were all more potent human 5HT2B agonists than MDMA or MDA. Cultured human heart valve interstitial cells (hVICs) incubated for 48 h with 10 mcM fenfluramine, norfenfluramine, MDMA or MDA all showed signs of mitogenesis (cell growth), but only fenfluramine and norfenfluramine stimulated statistically significant mitogenesis (compared with vehicle). The 5HT2B antagonist SB206553 (at 1 mcM) failed to stimulate cell growth and halted cell growth in cultures incubated with fenfluramine, norfenfluramine, MDMA or MDA. Other studies of transgenic mice are summarized but not described in detail, with these studies suggesting that the 5HT2B receptor is essential for heart tissue development. As expected, an assay of EC(50) for monoamine release found that MDMA releases serotonin (5-HT), dopamine (DA) and norepinephrine (noradrenaline) (NE) from rat synaptosomes. However, surprisingly, MDA was found to be a more potent NE releaser than MDMA. The receptor profile of MDMA also suggests that MDMA has equal or greater activity at receptors other than monoamine transporter, such as specific alpha (norepinephrine) receptors and muscarinic (acetylcholine) receptors. Findings indicate that because they may stimulate heart cell growth, drugs that are 5HT2B agonists may also pose a risk for valvular heart disease. Hence MDMA and MDA may pose a risk for valvular heart disease in humans in a manner similar to fenfluramine. However, VHD appearing after fenfluramine use is rare, with reported prevalence varying from none detected to 12% (Davidoff et al. 2001; Rothman and Baumann 2002) and when it occurs, VHD appears only after chronic, daily fenfluramine use. MDMA is rarely, if ever, used in this manner, and so the actual risk posed by intermittent Ecstasy or MDMA use may be lower than estimated from these study findings. It should also be noted that MDA is only a minor MDMA metabolite, and that other compounds, like HMMA, are major metabolites (e.g. De la Torre et al. 2000; Ortuno et al. 1999; Pizarro et al. 2002), and it is uncertain whether HMMA has 5HT2B agonist activity.