Phocid seals are one of the few groups of mammals capable of sustaining the energetic demands of lactation entirely through body nutrient stores while fasting. Lactation performance of the female in turn influences the rate and pattern of pup growth. We examined variation in and patterns of milk composition and production, maternal energy output, and pup growth and energy deposition over the entire lactation period in 18 grey seal mother-pup pairs using hydrogen isotope (3H2O and D2O) dilution. Milk composition was independent of maternal mass and nutrient stores, indicating dependence on other physiological and genetic factors. Heavier females lactated longer (r2=0.653, P<0.001), had higher total milk outputs (r2=0.652, P<0.001), and produced larger pups at weaning (r2=0.417, P=0.005). While fatter females lactated for longer periods of time (r2=0.595, P<0.001), females with a larger lean body mass at parturition produced more milk (r2=0.579, P<0.001). Total milk energy output was the strongest predictor of pup weaning mass, which, along with the pup's efficiency of energy storage, accounted for 91% of the variation in weaning mass. Nevertheless, there was sufficient plasticity in milk composition and energy output that some smaller females produced relatively large pups. Few females appeared to deplete body nutrients to the point where it might limit the duration of lactation.