Specialised transporter proteins that are the products of two closely related genes, UT-A (Slc14a2) and UT-B (Slc14a1), modulate the movement of urea across cell membranes. The aim of this study was to characterise the mouse variants of two major products of the UT-A gene, UT-A1 and UT-A2, both of which are important components of the urinary concentrating mechanism. Screening a mouse kidney inner medulla cDNA library yielded 4047 and 2876 bp cDNAs, the mouse homologues of UT-A1 and UT-A2. Expression in Xenopus oocytes showed that, characteristic of UT-A family members, the cDNAs encoded phloretin-inhibitable urea transporters. Northern analysis using tissue obtained from humanely killed MF1 mice showed high expression of UT-A mRNAs in kidney medulla. UT-A transcripts were also present in testis, heart, brain and liver. By primer extension analysis and 5Ì RACE we determined the transcriptional start sites for UT-A1 and UT-A2. Independently, both techniques revealed that transcription of the mUT-A2 mRNA transcript is initiated from a single start site, which is distinct from the three transcriptional start sites that initiate transcription of UT-A1, and UT-A3. Western analysis using an antiserum raised to the 19 COOH-terminal amino acids of rat UT-A1 identified immunoreactive proteins in all tissues analysed, with a complex pattern of differential expression. Relative to other tissues, kidney and brain had the highest levels of UT-A protein expression. In kidney sections immunocytochemistry revealed immunoreactive proteins in type 1 and type 3 thin descending limbs of the loop of Henle and in the middle and terminal inner medullary collecting ducts. In conclusion, we have isolated and characterised two cDNAs encoding mouse UT-A1 and UT-A2.