In this issue of Heart Rhythm, Wu and colleagues1 investigated the role of serotonin and serotonin-related metabolites in the pathogenesis of vasovagal syncope. The understanding of the role of serotonin in the vasovagal reflex began in 1867 when von Bezold and Hirt reported that injections of veratrum alkaloids, which are extracted from Veratrum californicum and Veratrum viride, into animals caused hypotension, bradycardia, and apnea.2 These alkaloids have several pharmacodynamic effects, and many decades later, the hypotension-bradycardia reflex was found to be due to serotonin.