Serotonin (5-hydroxytryptamine, 5-HT) is an important neurotransmitter which mediates numerous physiological functions. Using the SacI-EcoRI restriction fragment of the rat brain 5-HT2 receptor cDNA as a probe, we have screened a mouse brain cDNA library, created by random priming and constructed in SWAJ vectors, and have isolated a cDNA encoding a 1.4 kb open reading frame which codes for a functional mouse 5-HT2 receptor identified from pharmacological binding profiles and coupling of phosphoinositide formation in a stably transfected fibroblast cell line. The deduced amino acid sequence is 97.4% identical to the rat 5-HT2 receptor. Using the same 5-HT2 receptor cDNA probe, ten positive genomic clones were isolated from two mouse genomic libraries constructed in the pWE15 cosmid vector and the EMBL-3 phage vector. Extensive mapping and sequencing of these genomic clones indicate the mouse 5-HT2 receptor coding region spans over 20 kb and is composed of three exons split by two introns. Northern blot analysis shows one band of 5-6 kb in the mouse brain, but not in the heart, lung, liver, or kidney total RNA. Southern analysis of mouse liver genomic DNA shows a simple pattern of digestion by several restriction enzymes, which suggests that one copy of the 5-HT2 receptor gene may exist in the mouse genome.