Maximum urine-concentrating capacity (UCC) differs widely among mammals of different species, being very high in some desert species (e.g. kangaroo rats) and very low in freshwater aquatic species (e.g. beaver). In this study, kidneys of 21 species of mammals from widely different habitats were studied in histological sections to determine whether differences in UCC can be attributed to differences in kidney structure. Parameters studied included the ratio of medullary to cortical thickness, the proportional subdivision of the medulla into inner and outer zones, and the dimensions of the vasa recta expressed in terms of the total area and the number of lumens within the vascular bundles. Determinations were made at a level where the size of individual vasa recta bundles has reached a constant maximum size, i.e. in the distal half of the outer zone. A positive correlation was found between the UCC and the ratio of medullary length to cortical thickness. No clear correlation existed between the proportion of the medullary length comprised of outer or inner zones and the UCC, although a trend to higher UCC in animals with relatively longer inner zones was apparent. Thus, it appears that the relative length of the entire medullary region is a major factor determining UCC, but the length of individual medullary zones is of lesser importance. A correlation was also found between the density of vasa recta per cubic millimeter of medullary tissue (the number of lumens regardless of identity in bundles, based on the number counted at the level sampled) and the UCC of the species. Data reported here support the view that UCC can be correlated with two parameters of kidney structure – the length of medulla relative to that of cortex and the density of vasa recta within the outer zone. It is proposed that the anatomical characteristics of the vascular supply to the medulla – that is, the vasa recta – are equally as important for the concentration of urine as is the primary mechanism determined by the characteristics of the loop of Henle and collecting