From bacteria to mammals, specialized membrane proteins regulate the transport of urea across cellular membranes. In mammals these proteins are derived from either the UT-A (Slc14a2) or the UT-B (Slc14a1) genes. Proteins arising from both genes show a broad distribution throughout the body indicative of multiple physiological roles. In this presentation I will focus on UT-A urea transporters with special emphasis on recent findings. The structure of the UT-A gene has been resolved in mouse, rat and human and resides on Chromosome 18 q12.3. Not surprisingly, these three genes share several characteristics. These include; their large size (all of which are greater than 400kb); they reside in tandem on Chromosome 18 with the UT-B gene; their transcription is driven by two promoters; and they give rise to multiple mRNA isoforms as a result of differential promoter activity, alternative splicing and the use of alternate transcription and polyadenylation signals. To date five protein orthologs have been characterized that originate from the UT-A gene. UT-A mRNAs and proteins have been detected in several tissues including testis, vas deferens, and colon [1], however the tissue showing the greatest abundance, both in terms of the number of different isoforms present and level of expression, is the kidney. In the mouse and rat kidney, isoforms UT-A1, UT-A2 and UT-A3 are the most abundant. UT-A1 is the largest UT-A isoform with 97 and 117kDa glycosylated protein forms. UT-A2 consists of the carboxyl terminal 397 amino acids of UT-A1 and exists as 43-55kDa glycosylated proteins. This isoform has been localized to type 1 and type 3 thin descending limbs (tDL) of the loop of Henle. In these nephron segments it is proposed to participate in intra-renal recycling of urea between the IMCD and loop of Henle. UT-A3 consists of the amino-terminal 460 amino acids of UT-A1 and occurs as 44 and 67 kDa glycosylated proteins. Both UT-A1 and UT-A3 localize to the inner medullary collecting duct (IMCD) of the kidney where they regulate urea reabsorption into the interstitium under the acute control of arginine vasopressin (AVP). Through the creation of UT-A deficient mouse strains we and others have highlighted the central role of UT-A proteins in the urinary concentrating mechanism and this work has led to the reevaluation of several long standing hypotheses namely those advanced by Berliner [2], Kokko and Rector [3] and Stephenson [4]. Whereas mice devoid of UT-A2 show only a relatively mild defect in the urinary concentrating mechanism [5], mice lacking UT-A1 and UT-A3, have a severe concentrating defect. Interestingly, the severity of this phenotype is reciprocally related to the protein content of the diet [6] To study the function and regulation of UT-A transporters at the cellular level, we have recently engineered MDCK type I cell lines to stably express mouse UT-A proteins. These cell lines, when grown on permeable supports, have enabled urea transporter function to be assessed via radiolabelled urea flux across a polarized epithelia cell monolayer. This approach has successfully been used to study acute regulation of UT-A1-, UT-A2- and UT-A3-mediated urea transport. In addition, we have also developed novel strategies to study urea transporter function. By making membrane vesicles from Xenopus oocytes expressing UT-A transporters and using light scattering techniques we have been able, for the first time, to measure the kinetics of UT-A transporter isoforms [7]. Finally, using mutagenesis and by making UT-A protein chimeras we are beginning to probe the structure of the UT-A proteins. This in combination with studies to optimize heterologously expression of bacterial and mammalian urea transporters should in the near future enable us to unlock the tertiary structure of these uniquely important molecules.