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A laboratory study investigated the metabolic physiology, and response to variable periods of water and sodium supply, of two arid-zone rodents, the house mouse (Mus domesticus) and the Lakeland Downs short-tailed mouse (Leggadina lakedownensis) under controlled conditions. Fractional water fluxes for nl. domesticus (24 +/- 0.8%) were significantly higher than those of L. lakedownensis (17 +/- 0.7%) when provided with food ad libitum. In addition, the amount of water produced by M. domesticus and by L. lakedownensis from metabolic processes (1.3 +/- 0.4 ml . day(-1) and 1.2 +/- 0.4 ml . day(-1), respectively) was insufficient to provide them with their minimum water requirement (1.4 +/- 0.2 ml . day(-1) and 2.0 +/- 0.3 ml . day(-1), respectively). For both species of rodent, evaporative water loss was lowest at 25 degrees C, but remained significantly higher in M. domesticus (1.1 +/- 0.1 mg H2O . g(-0.122).h(-1)) than in L. lakedownensis (0.6 +/- 0.1 mg H2O . g(-0.122) . h(-1)). When deprived of drinking water, mice of both species initially lost body mass, but regained it within 18 days following an increase in the amount of seed consumed. Both species were capable of drinking water of variable saline concentrations up to 1 mol . l(-1), and compensated for the increased sodium in the water by excreting more urine to remove the sodium. Basal metabolic rate was significantly higher in M. domesticus,s. (3.3 +/- 0.2 mg O-2 . g(-0.75) . h(-1)) than in L. lakedownensis (2.5 +/- 0.1 mg O-2 . g(-0.75) . h(-1)). The study provides good evidence that water flux differences between M. domesticus and L. lakedownensis in the field are due to a requirement for more water in M. domesticus to meet their physiological and metabolic demands. Sodium fluxes were lower than those observed in free-ranging mice, whose relatively high sodium fluxes may reflect sodium associated with available food.