The polymerase chain reaction has been employed to isolate a cDNA encoding a functional 5-HT3 receptor subunit from the murine neuroblastoma cell line N1E-115. Overall, the amino acid sequence predicted from this clone demonstrates a 98% homology with the 5-HT3 receptor A subunit cloned from NCB-20 hybridoma cells. A deletion of 6 amino acid residues located within the putative large intracellular loop, which may result from alternative splicing, represents the principal difference between the two clones. Upon expression in Xenopus oocytes, the homo-oligomeric receptor displayed pharmacological properties which define it as a functional 5-HT3 receptor.