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Dugongs (Dugong dugon) are fully marine mammals that live independently of fresh water so must balance water and electrolytes in a hyperosmotic environment. To investigate osmoregulation, matched plasma and urine from 51 live wild dugongs were analysed for osmolality, major electrolytes (Na + , Cl - , K + ), urea, creatinine, and glucose. Maximum urine osmolality (1468 mOsm kg -1 ) and Na + , K + , and Cl - concentrations (757, 131.3, 677 mmol L -1 , respectively) indicate that dugongs are capable of concentrating urine above seawater and could potentially realise a net gain of free water from drinking seawater. However, mean urine osmolality of 925.4 (± 46.6) mOsm kg -1 suggests that mariposia is unlikely to be an important osmoregulatory mechanism. Dugongs may obtain enough preformed water from their seagrass diet and metabolic oxidation to maintain homeostasis. Mean plasma osmolality of 339.6 (± 1.8) mOsm kg -1 is higher than in the related manatees but within the range for fully marine cetaceans. Relatively high mean plasma Na + (175.5 ± 1.7 mmol L -1 ) and K + (6.9 ± 0.1 mmol L -1 ), as well as mean urinary Na + (469.6 ± 22.5 mmol L -1 ) and K + levels (32.5 ± 4.5 mmol L -1 ) may reflect a salt-rich seagrass diet. Pregnant females had higher mean plasma osmolality (355.3 ± 4.9 mmol L -1 ) than non-pregnant females and males (337.9 ± 1.7 mOsm kg -1 ), suggesting that fluid retention was not a feature of pregnancy. Further research on water intake and endocrinology will enhance our understanding of osmoregulation in dugongs.