The somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF binding proteins (IGFBP), is a bridge between growth physiology, developmental age, and nutritional status in domestic animals. However, the importance of the somatotropic axis in nutrition, growth, and development of harbor seals has not been previously explored. Given the difficulty of conducting longitudinal studies in free-ranging harbor seals, this study focused on the potential use of harbor seals in rehabilitation facilities as a model for free-ranging seals. The purpose of this research was to compare concentrations of components of the somatotropic axis in free-ranging versus rehabilitated harbor seal pups. The hypothesis was that measurements of the somatotropic axis will be similar between individuals of comparable age and nutritional status (fasting versus feeding). To investigate this hypothesis, harbor seal pups (n = 8) brought to The Marine Mammal Center (Sausalito, California, USA) or Mystic Aquarium (Mystic, Connecticut, USA) were initially assessed and determined to be healthy but abandoned. All pups were less than 2 wk of age upon arrival at rehabilitation facilities. Standard length was assessed at the time of arrival and again at release. Body mass was measured every week and blood samples were collected from each pup at 0, 4, and 8 wk of rehabilitation. Blood was collected and morphometrics assessed in free-ranging harbor seal pups (n = 8) from the Gulf of Maine. Sera were analyzed for GH, IGF-I, and IGFBP concentrations. Concentrations of GH, IGF-I, and IGFBP-2 and -3 in rehabilitated pups were within a similar range compared with free-ranging pups when considered in the context of presumed nutrient intake. These data suggest that rehabilitated harbor seals may provide a useful model to investigate the effects of nutrient intake on growth and development of harbor seals, and will provide insight into phocid endocrinology and metabolism.