This study measured resting metabolic rate (RMR), lean body mass (LBM) and fat body mass (FBS) in dogs of 3 different breeds (Papillon, Labrador retriever and Great Dane) and ages between 0,6 and 14,3 years. LBM increased with age in Papillons, and remained constant in Labrador retrievers and Great danes. FBM remained constant with age in Papillons and Labrador retrievers, and increased in Great Danes. RMR was positively correlated with LBM and FBM and negatively correlated with age. However, RMR is influenced by breed, and is lower in Labrador retrievers compared to the other 2 breeds, while energy expenditure and life expectancy were higher in the smallest breed.