To elucidate ecological effects of variation in the temporal distribution of a limiting resource (water in the Mojave Desert), energetics of two free—living populations of desert tortoises (Gopherus [Xerobates] agassizii) were studied concurrently over 18 mo with use of doubly—labeled water. Field metabolic rates (FMR) and feeding rates (estimated from rates of water influx and rates of change in dry mass) were highly variable. This variability was manifested at several levels, including seasonal changes within populations, year—to—year differences within populations, and differences between populations. Underlying observed patterns and contrasts was considerable variation among individuals. Much of the variation in energetic variables was associated with a single climatic variable, rainfall. Seasonal, annual, and interpopulation differences in FMR and foraging rates corresponded to differences in availability of free—standing water from rainstorms. At least some of the differences among individuals were apparently due to differences in proclivity or ability to drink. Tortoises had very low FMRs relative to other reptiles, which allowed them to tolerate long periods of chronic energy shortage during a drought. Calculations suggested that tortoises experienced a net loss of energy on their spring diet of succulent annual plants. If so, tortoises require drier forage to accrue an energy profit, which emphasizes their reliance on drinking rainwater (which can be stored in the bladder and resorbed later to hydrate dry forage). Further, it suggests that growth (as protein deposition) and net acquisition of energy may be temporally decoupled in desert tortoises, which has potential consequences for geographic variation in life history traits. Energy acquisition and expenditure in desert tortoises are thus strongly constrained by the contingencies of rainfall, both indirectly through effects on availability and quality of food, and directly through reliance on free—standing water for drinking, which is apparently necessary for achieving a net annual energy profit.