Cetaceans (whales and dolphins) always excrete urine with an osmolality markedly higher than that of plasma. Although the mechanisms by which cetaceans concentrate urine have not been elucidated, data support a role for medullary urea accumulation in this process, as is the case for terrestrial mammals. Therefore, we hypothesized that facilitated urea transporters are present in the kidney of cetaceans. Using 5'/3'-rapid amplification of cDNA ends, we cloned a 2.7-kb cDNA from the kidney of the short-finned pilot whale Globicephala macrorhynchus. The putative open-reading frame encoded a 397-amino acid protein [pilot whale urea transporter A2 (whUT-A2)] that has 94% amino acid sequence identity to the A2 isoform of the human urea transporter (hUT-A2). Heterologous expression of whUT-A2 cRNA in Xenopus oocytes induced phloretin-inhibitable urea transport. Although Northern analysis and RT-PCR indicated that whUT-A2 was exclusively expressed in kidney, Western blotting using a polyclonal antibody to rat UT-A1/UT-A2 detected various immunoreactive proteins in kidney and other tissues. Furthermore, RT-PCR analysis suggested the presence of alternatively spliced UT-A transcripts in the kidney as well as extrarenal tissues. We conclude that renal urea transporters are highly conserved among mammals inhabiting terrestrial and pelagic environments. A urea-based concentrating mechanism, presumably evolved to meet the demands of an arid terrestrial environment, may have contributed a fortuitous preadaptation that enabled the ancestors of cetaceans to reinvade the sea.