The energetics of lactation in grey seal females and their pups was investigated using labelled water methods. Average total energy output (maternal maintenance energy expenditure plus milk energy output), from the estimated change in total body gross energy, of 15 females was 99.8 MJ/d (S.E. 2.5), which was nearly 30% lower than previously published estimates. In seven females, energy output was partitioned into milk energy output plus maternal maintenance energy expenditure. Average investment in milk was 69.7MJ/d (S.E. 5-0), and lactation costs accounted for 70% of total maternal energy output. Average daily maternal maintenance energy expenditure (heat output) of the same seven females was 30.0MJ/d (S.E. 5.0), which was equivalent to 2.3 times the basal metabolic rate predicted from Kleiber's (1975) allometric equation. In seven pups, milk consumption rates were measured isotopically and these averaged 3.0kg/d (S.E. 0.2). In four pups energy expenditure, measured using doubly-labelled water, averaged 13.5MJ/d (S.E. 1.2), equivalent to 3.7 times the basal metabolic rate predicted from Kleiber's (1975) equation. All females were in negative water balance during lactation which was associated with loss of tissue during fasting. Mean water efflux was 3966ml/d (S.E. 286) and mean water influx was 2677ml/d (S.E. 245), but there was no evidence that females are dehydrated during lactation.