The field metabolic rates (FMR) and water fluxes of Varanus scalaris were measured during the wet and dry seasons by the doubly-labelled water technique. Seasonal measurements of standard (night-time) metabolism (SMR) and resting (daytime) metabolism (RMR) were made in the laboratory at 18, 24, 30 and 36°C, and maximal oxygen consumption was measured at 36°C on a motorized treadmill. This population was active throughout the year. In the wet season, the mean FMR was 7.8 kJ day−1 (128.0 kJkg−1 day−1; mean mass = 66.4 g, n= 13), and during the dry season the mean was 5.0 kJ day−1 (67.6 kJ kg−1 day−1; mean mass = 77.4 g, n= 17). The mean water flux rates for these animals were 3.6 and 1.2 ml day−1, respectively (60.4 and 16.6 ml kg−1 day−1). The seasonal means of FMR and water flux were significantly different by ANCOVA (P < 0.0001). Measurements of SMR and RMR were significantly higher in the wet season (ANCOVA: P < 0.0001), but we found no difference in the maximal oxygen consumption between seasons (ANCOVA: P= 0.6). The maximal oxygen consumption of the lizards on the treadmill (2.9 ml min−1= 1.8 ml g−1 h−1), mean mass = 97.4 g, n= 16) was 20 times that of the SMR at the same temperature during the dry season, and 11 times that of the SMR during the wet season. The seasonal differences in FMR were attributable to: changes in SMR (12.2%) and RMR (16.4%); differences in night-time body temperatures (11.3) and daytime body temperatures (16.4%); and activity (broadly defined to include locomotion, digestion, and reproductive costs (43.7%).