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We compared the efficiency of water regulation of two rodent species - Phyllotis xanthopygus rupestris and Phyllotis darwini, inhabiting contrasting habitats in South America. We evaluated water requirements ad libitum, resistance index to water deprivation (water deprivation test), evaporative water loss, and renal morphology. Water consumption of P. x. rupestris was significantly higher than in P. darwini, P. x. rupestris and P. darwini subjected to water deprivation, showed a negative relationship between body mass loss and water deprivation time. Minimal evaporative water loss values were not significantly different between species. Although P. x. rupestris and P. darwini were not able to maintain their body mass under water deprivation tests, they have different water economies. This suggests that due to their distinct inhabitats with contrasting water availability, they have different adaptative strategies in their respective environments. (c) 2007 Elsevier Ltd. All rights reserved.