This invention provides an isolated nucleic acid molecule encoding a mammalian 5-HT4B receptor and an isolated nucleic acid molecule encoding a human 5-HT4B receptor, an isolated protein which is a mammalian 5-HT4B receptor, an isolated protein which is a human 5-HT4B receptor, vectors comprising an isolated nucleic acid molecule encoding a mammalian 5-HT4B receptor, vectors comprising an isolated nucleic acid molecule encoding a human 5-HT4B receptor, mammalian cells comprising such vectors, antibodies directed to the 5-HT4B receptor, nucleic acid probes useful for detecting nucleic acid encoding a mammalian or human 5-HT4B receptor, antisense oligonucleotides complementary to any sequences of a nucleic acid molecule which encodes a mammalian or human 5-HT4B receptor, pharmaceutical compounds related to the human 5-HT4B receptor, and nonhuman transgenic animals which express DNA encoding a normal or a mutant mammalian or human 5-HT4B receptor. This invention further provides methods for determining ligand binding, detecting expression, drug screening, and treatments for alleviating abnormalities associated with a human 5-HT4B receptor.