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Extremely premature infants, born at 28 weeks gestation or less, are at greatest risk for poor neurodevelopmental outcomes. While survival of these infants has improved in the past decade, neurodevelopmental outcomes have not. Because early life experiences affect brain structure and function, the quality of these experiences is one of the most important factors affecting optimal development. Reliable markers of neurobiological processes underlying development are necessary so that research can accurately monitor mediators of neurodevelopmental outcomes. Oxytocin (OT) has the potential to be a neurobiological marker of social processes that offer neuroprotection for the infant. OT acts as a buffer for the stress response system and provides protection to the brain during inflammation, ischemia, or injury. OT has been strongly linked to neurodevelopmental outcomes in animal models, particularly those outcomes related to social cognition and emotion regulation. No studies measuring OT have been conducted in premature infants, nor has the association of oxytocin levels and neurodevelopment for these infants been investigated. The purpose of this study is to 1) describe OT levels in plasma, urine, and saliva in premature infants through 34 weeks gestation and 2) determine if OT levels vary with maternal-infant interaction, neurobehavioral organization, and infant stress exposure. Thirty-seven premature infants, born gestational ages 25-28 6/7 weeks, were longitudinally followed until 36 weeks gestation. Plasma and urine samples were collected at 14 days of life, then weekly until 34 weeks. Data on infant and environmental variables were abstracted from the electronic medical record. Infant social engagement behaviors was measured by the Parent-Child Early Relational Assessment, during a videotaped feeding when the infant was at one-quarter full oral feeds. Infant stress exposure was measured weekly by the Neonatal Infant Stressor Scale. Neurobehavioral organization was measured by the NICU Network Neurobehavioral Scale at 36 weeks gestation. Plasma OT levels significantly decreased with age, at a rate of 15% per week. Urine OT levels did not significantly change with age. However, more research is needed before concluding that urine is not an acceptable noninvasive measurement in this population. Both plasma and urine exhibited wide variability across age, but values were significantly stable within infants. Plasma and urinary OT levels were not correlated, both within and between infants. Moreover, OT levels were not related to infant social engagement behaviors or infant neurobehavioral organization. We hypothesize that the stressful nature of the NICU environment may contribute to decreasing OT in premature infants. Future research must replicate these results, as well as determine how stress and the NICU environment impact OT levels in premature infants. We also hypothesize that before the emergence of coordinated movements and behaviors, premature infants primarily socially interact with their caregivers through their physiology. Future research should investigate associations among the physiologic coregulation of a dyad, coregulation of dyadic behaviors, and infant neurodevelopment. OT may serve as an important biomarker when investigating the window of development that encompasses the infant’s transition from the biologic to the social world.