We describe a genomic clone encoding the human 5-HT1B receptor. This apparently intronless gene encodes a 390 amino acid polypeptide homologous to the rat 5-HT1B serotonin receptor, with which it shares 93% amino acid sequence identity. Remarkably, [3H]5-hydroxytryptamine binding studies with transfected HeLa cells show that the human 5-HT1B receptor has a pharmacological profile that is markedly different from that of the corresponding rat receptor. Instead, human 5-HT1B drug specificity is highly similar to that of the human 5-HT1D receptor, with which it shares 59% amino acid sequence identity. The human 5-HT1B receptor, like the 5-HT1D receptor, can couple to Gi proteins. The presence of the threonine355 in the human receptor rather than an asparagine, as found in the corresponding rat gene product, may explain much of the marked pharmacological difference between the human and rat 5-HT1B receptors.