Deuterium oxide was used to measure isotope dilution space 11 times in seven free living, adult, male Atlantic walruses (Odobenus rosmarus rosmarus) (930–1597 kg) in north-east Greenland in the month of August 2000 and August 2001. Telemetry instruments, which were attached to the animals as part of a long term study, provided data on activity levels. Equilibration of the intravenously injected isotope was complete after 2–3 h. The application of a general correction factor developed for all pinniped species allowed the estimation of total body water content as 56.8% (range: 36.6–73.3%) of total body mass (TBM). Water turnover averaged 44.5 g kg-1 d-1. For one animal it was possible to estimate water influx to 4.8 g kg-1 d-1 and water efflux to 21.8 g kg-1 d-1. The estimates of body fat and body protein averaged respectively 24.7% and 18.4% of TBM. Similarly, for an ‘average’ walrus with a TBM of 1300 kg body ash was calculated to average 26 kg and body gross energy to 18,300 MJ. The assessment of body fat by isotope dilution did not differ significantly from the estimates of blubber content obtained from the dissection of walruses in the wild.