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The doubly-labelled water technique was used to measure energy expenditure in 20 free-living kakapo (Strigops habroptilus) on Codfish and Little Barrier Islands. Daily energy expenditure (DEE) averaged 799 kj/d, equivalent to 1.4 x BMR (basal metabolic rate), the lowest value recorded for any adult wild bird. DEE was higher in males than females, and was greater on Codfish Island than on Little Barrier Island. Supplementary food taken from hoppers by kakapo supplied about half of their DEE; a few individuals apparently obtained virtually all their energy needs from supplementary food. Use of food from hoppers did not affect energy expenditure directly, but apparently did so via long-term elevation of body mass. Supplementary feeding, particularly of energy-dense items such as nuts and seeds, greatly depressed body-water turnover rates. Some implications of the often high level of supplementary food taken by kakapo are discussed. Adjusting the supplementary feeding programme to meet more precisely the needs of individual birds would probably improve the overall nutrition of the surviving kakapo population.