All the Namib rodents investigated, except Petromus, can survive without water on a diet of air-dried seed. Water turnover rates of these animals in the laboratory confirmed their low water requirements. Field water turnover rates were remarkably low, probably because Petromyscus and Aethomys aestivate and Petromus has a low daily energy expenditure (DEE). The rodents have an increased water turnover rate after advective fogs occur: Daily energy requirements for Petromyscus and Aethomys were similar to expected values in the laboratory, whereas Gerbillurus had a higher DEE, and Petromus and Mus had lower-than-predicted DEE's. Water turnover rate is coupled with energy turnover rate for the Namib rodents. These physiological attributes of the Namib rodents are consistent with the severity of their environment, are associated with their kidney structure, and reflect the evolutionary history and demography of the particular species.