Water availability is a growing concern for livestock farmers due to climate change, particularly in arid regions and increasingly in temperate regions. Water use efficiency may become an important trait to select for in temperate breeds, thus the objective here was to evaluate the water balance and body water dynamics of Lleyn ewes. Ewes (n = 15), assigned to one of three isotopically distinct drinking waters (DW) with delta H-2 values of -49 parts per thousand, + 241 parts per thousand and + 1143 parts per thousand, were housed individually and fed at maintenance for 82 days. Total water (TW) intake was recorded daily and water loss via urine and faeces was determined daily during four campaign weeks. The average TW intake was 2.7 +/- 0.63 kg d(-1) with DW contributing 86.3 +/- 1.98%. There was a strong correlation between DW intake and temperature-humidity index (r = 0.77; p < 0.01). The average measured water loss was 39.4% of TW intake, indicating a large unmeasured route of water loss, likely expiration. Time-series isotopic analysis of blood and urine waters revealed average half-lives of 13.2 +/- 4.09 d and 14.4 +/- 5.18 d, respectively, and they were not statistically different. There was a three-fold variation in half-lives among ewes, demonstrating that there is potential to increase the water use efficiency in temperate sheep breeds, aligned to future breeding goals to improve sustainability traits. An isotope mixing model was established to predict equilibrium body water delta H-2 values, but future work is required to refine the model to account for H-2 incorporation into body tissues.