Captive breeding and release is a tool used by conservation biologists to re-establish populations of endangered or locally extinct species. Reintroduced animals that have been bred in captivity must learn to meet the challenges posed by free living, and to adjust to local environmental conditions, food and water sources. How well reintroduced animals might meet these challenges is uncertain as few longitudinal studies have investigated the physiology of reintroduced animals or the implications of this for successful establishment of new populations. Here we have evaluated long-term, seasonal energy and water use by reintroduced yellow-footed rock-wallabies (Petrogale xanthopus celeris), an endangered medium-sized marsupial that inhabits rocky outcrops across Australia's arid and semiarid rangelands. Captive-bred rock-wallabies were reintroduced to an area within the known boundaries of their former range, in south-western Queensland, Australia. Post-release water turnover rates (WTR) and field metabolic rates (FMR) were measured during their first wet summer and dry winter, by means of the doubly labelled water method. Total body water (73.1%), FMR (1650.0 kJ day-1), female fecundity (100%), and male and female body masses and survival were consistent between seasons, but rates of water turnover were significantly lower for all animals during the dry winter (174.3 mL day-1) than during the wet summer (615.0 mL day-1). There were no significant differences in WTR or FMR between males and lactating females (in either season).