Previous studies have demonstrated the presence of mitogenic serotonin [i.e., 5-hydroxytryptamine (5-HT)] receptors on glomerular mesangial cells and have linked those receptors to a complicated array of intracellular and autocrine/paracrine signaling pathways [T. Knauss and H. E. Abboud. Am. J. Physiol. 251 (Renal Fluid Electrolyte Physiol. 20): F844-F850, 1986; and N. Takuwa, M. Ganz, Y. Takuwa, R. B. Sterzel, and H. Rasmussen. Am. J. Physiol. 257 (Renal Fluid Electrolyte Physiol. 26): F431-F439, 1989]. Those studies suggested that the mesangial subtype of 5-HT receptor is a member of the 5-HT2 receptor family, which consists of three known members, designated as subtypes A, B, and C. The purpose of the current study was to identify the subtype of 5-HT2 receptor present on mesangial cells. Northern blot showed detectable mRNA for a putative 5-HT2A receptor, but not for 5-HT1A or 5-HT2C receptors. Reverse transcription-polymerase chain reaction (RT-PCR) with degenerate oligonucleotides derived from the putative third and sixth transmembrane domains of cloned 5-HT2 receptors yielded a 580-nucleotide (nt) fragment. RT-PCR with primers highly specific for the 5-HT2A receptor and designed to amplify > 95% of its coding block yielded a product of 1,320 nt. Nested PCR reactions yielded products of the predicted sizes for the 5-HT2A receptor. Partial sequence information was obtained, and the sequence corresponded exactly (627/627 nt) to that published for the cloned rat brain 5-HT2A receptor. These studies identify the mesangial cell mitogenic 5-HT receptor as a 5-HT2A receptor subtype.