A study was made of the energetics of the Bank vole, Clethrionomys glareolus, on a number of different diets. Daily energy flow was shown to have considerable variation on the different diets examined; oats, wheat, acorns and bluebell corms. Energy flow of voles living in pairs was calculated and shown to have lower values than for voles living singly. The energetics of weight change were also investigated. Approximate values were obtained for daily energy flow of voles in the wild and these were compared with the values determined in the laboratory. Voles in the wild were calculated to have much greater values for energy flow.