The energetics of lactation in domestic dogs was investigated. Two breeds were chosen of different masses, Labrador Retrievers (30kg) and Miniature Schnauzers (6kg). Labradors had larger litter sizes and invested more energy relative to adult weight during lactation. This was shown in a number of ways. During peak lactation they had relatively higher metabolised energy intakes, higher energy expenditures, higher ratios of litter metabolic mass to maternal metabolic mass and produced relatively more milk. In addition Labradors utilised body reserves, an investment the Schnauzers didn't make. Schnauzers, however, invested relatively more energy per pup, which was reflected by their relatively larger offspring energy expenditures. Offspring of both breeds increased significantly in mass over the experimental period of peak lactation, although the Labrador pups were significantly leaner and contained less fat in proportion to body mass than the Schnauzer pups. The gradient of the scaling relationship between litter size and body mass across different breeds of dog was significantly steeper and tighter than the interspecific scaling relationship calculated between different species of Canid of similar mass range. This is consistent with the more recent theories on the evolution of life histories, that interspecific allometric variation is an epiphenomenon of intraspecific variation. This suggests that physical constraints associated with size in dogs may limit offspring investment and milk production. The differences in energy investment during lactation between the two breeds might be explained by variations in their ability to sustain high energy expenditures. Central limitations to maximum metabolisable energy intake and energy expenditure might be important in determining reproductive output in the Labradors. Hence milk energy output might be limited by the ability of the alimentary system to acquire nutrients. In contrast, peripheral limitations might limit Schnauzers' ability to allocate energy to offspring. Hence milk production might be limited by mammary output capacity. A three dimensional model relating energy expenditure and fitness to body mass was produced. This advanced a number of predictions. For instance, it is predicted that the observed FMR/RMR ratio would be greater in larger animals. Second, that maximal sustained metabolic rate would be more likely to be limited peripherally in small mammals. Thus maximum energy intake required for single activities, such as lactation or thermoregulation, could represent peripheral limitations in small mammals but central limitations in large mammals.