Habitat preferences of rats, R. norvegicus and R. exulans, in housing areas were explained by renal morphologic indices of the ability to conserve urinary water. Indices involving medullary thickness (precent medullary thickness or PMT, ratio of medulla : cortex, and ratio of inner medullary zone : cortex) were significantly larger in R. exulans (for example, PMT±SD=73.2±1.7) than in R. norvegicus (69.4±2.8) at the 1% level. R. exulans thrives even in rural areas with poor water sources owing in part to the efficient kidneys to conserve water. R. norvegicus, on the other hand, flourishes only in urban areas, associated with abundant sources of water supply such as drains and a river.