The G protein coupled serotonin (5-HT) receptors, with seven membrane spanning domains, form a multigene family of which several members have been cloned and sequenced. The presence of 5-HT1D binding sites to our knowledge has not yet been reported in mouse. Here we describe the cloning and sequencing by the polymerase chain reaction (PCR) method of two 5-HT1D receptor sequences of the third cytoplasmic loop in mouse, strongly suggesting the existence of two 5-HT1D receptor genes, located on chromosome 4. A homologous sequence to one of them was cloned in guinea pig.