- 1. Desert gerbilline rodents consume mainly plant material, in particular seeds. However, seeds are less available following winter rains than in summer and many rodents shift their diets from predominantly seeds to ones containing more vegetation. This shift results in a proportionately greater preformed water intake and, as these rodents do not drink, an increase in the water influx to field metabolic rate ratio (WI : FMR; ml kJ–1). Thus, this ratio is indicative of the food consumed, being low when gerbils are consuming seeds and high when consuming green vegetation. 2. Smaller animals are more dependent on a highly digestible, high energy diet such as seeds and are more vulnerable to extreme air temperatures than are larger animals. It was predicted that: (1) gerbils consume less seeds in winter than in summer but the difference between seasons in seed intake is less pronounced in smaller than in larger rodents; and (2) smaller gerbils are more sensitive to extreme air temperatures than are larger gerbils. To examine these hypotheses, two coexisting desert gerbils, Gerbillus henleyi (8–12 g) and Meriones crassus (50–110 g), were studied. 3. It was predicted that: (1) the WI : FMR ratio would be greater in winter than in summer in both species, but the proportionate increase would be smaller in G. henleyi than in M. crassus; and (2) energy expenditure would be greater in winter than in summer in both species, but the proportionate increase would be greater in G. henleyi than in M. crassus. To test these predictions, winter and summer water influx and field metabolic rates in these species were measured under free-living conditions using doubly labelled water. 4. The WI : FMR ratio in summer was 0·048 in G. henleyi and 0·069 in M. crassus, indicating that G. henleyi was consuming proportionately more seeds than M. crassus. In winter, the ratio was 0·052 in G. henleyi, indicating that there was little change in dietary composition and that this species was still consuming mainly seeds. In contrast, the ratio in M. crassus rose to 0·127, indicating a shift to a diet containing more green vegetation. The relatively higher seed intake by G. henleyi than M. crassus in both seasons and the small, if any, seasonal change in seed intake in G. henleyi agrees with the hypothesis of body size and seed intake in gerbils. Winter FMR increased over summer FMR in both species, as predicted; however, the expected greater increase in G. henleyi relative to M. crassus did not occur. The winter FMR was 1·52 times the summer FMR in G. henleyi and 1·53 times the summer FMR in M. crassuss.