Serotonin (5-HT) was isolated and chemically characterized nearly four decades ago, and is now generally accepted to function as a neurotransmitter and neuromodulatory agent. Early research focused on the measurement of concentrations, synthesis, and metabolism of 5-HT, and only recently has the focus shifted to characterization of 5-HT receptors. Gaddum and Picarelli first sug gested in 1957 that the effect of 5-HT in the guinea pig ileum is mediated by two pharmacologically distinguishable receptors; however, the possibility of dual5-HT receptors was not explored systematically or...