Ontogeny of CORT levels. CORT levels in response to .5 mA foot-shock were measured in PN8, PN14, or PN21 pups with or without maternal presence to assess the ontogeny of social buffering. Immediately following 11 shock presentations with an interval of 4 min (between 12 and 2 pm), pups were anesthetized with pentobarbital and blood was taken from the hearts’ ventricle. Shock elicits a significant increase in CORT in PN14 and PN21 pups, but fails to do so in neonatal pups (PN8). Maternal presence in PN14 and PN21 pups prevents the CORT response to shock (Moriceau & Sullivan, 2006; Moriceau et al., 2006). The mother was anesthetized by urethane to prevent her from interfering with the shock administration, as well as to control for maternal behavior and milk letdown. The pups were, however, free to contact the mother. Stress =foot-shock; PN =postnatal day.