Urea transport in the kidney is important for the production of concentrated urine and is mediated by a family of transporter proteins, identified from erythropoietic tissue (UT-B) and from kidney (UT-A). Two isoforms of the renal urea transporter (UT-A) have been cloned so far: UT-A1 and UT-A2. We used rapid amplification of cDNA ends to clone two new isoforms of the rat UT-A transporter: UT-A3 and UT-A4. UT-A3 and UT-A4 are 87% homologous. The UT-A3 cDNA encodes a peptide of 460 amino acids, which corresponds to the amino-terminal half of the UT-A1 peptide and is 62% identical to UT-A2. The UT-A4 cDNA encodes a peptide of 466 amino acids, which is 84% identical to UT-A2. Transient transfection of HEK-293 cells with the UT-A3 or UT-A4 cDNA results in phloretin-inhibitable urea uptake, which is increased by forskolin. Thus, both new isoforms encode functional urea transporters that may be vasopressin-regulated. UT-A3 and UT-A4 mRNA are expressed in the renal outer and inner medulla but not in the cortex; unidentified UT-A isoforms similar to UT-A3 may also be expressed in the testis. It is concluded that there are at least four different rat UT-A urea transporters.