Arctic marine ecosystems are currently undergoing rapid environmental changes. Over the past 20 years, individual growth rates of beluga whales (Delphinapterus leucas) have declined, which may be a response to climate change; however, the scarcity of physiological data makes it difficult to gauge the adaptive capacity and resilience of the species. We explored relationships between body condition and physiological parameters pertaining to oxygen (O-2) storage capacity in 77 beluga whales in the eastern Beaufort Sea. Muscle myoglobin concentrations averaged 77.9 mg g(-1), one of the highest values reported among mammals. Importantly, blood haematocrit, haemoglobin and muscle myoglobin concentrations correlated positively to indices of body condition, including maximum half-girth to length ratios. Thus, a whale with the lowest body condition index would have similar to 27% lower blood (26.0 versus 35.7 ml kg(-1)) and 12% lower muscle (15.6 versus 17.7 ml kg(-1)) O-2 stores than a whale of equivalent mass with the highest body condition index; with the conservative assumption that underwater O-2 consumption rates are unaffected by body condition, this equates to a >3 min difference in maximal aerobic dive time between the two extremes (14.3 versus 17.4 min). Consequently, environmental changes that negatively impact body condition may hinder the ability of whales to reach preferred prey sources, evade predators and escape ice entrapments. The relationship between body condition and O(2 )storage capacity may represent a vicious cycle, in which environmental changes resulting in decreased body condition impair foraging, leading to further reductions in condition through diminished prey acquisition and/or increased foraging efforts.