The question of whether a sexually dimorphic stress reaction exists prior to extensive socialization was addressed by examining sex differences in physiological and behavioral stress reactivity, in healthy, term neonates, after a mildly stressful behavioral assessment procedure. The Neonatal Behavior Assessment Scale (NBAS; Brazelton, 1973) was administered to 18 male and 18 female neonates. Heart rate (HR), salivary cortisol, and behavioral states were assessed before and after the exam. Sex differences included higher cortisol response in males and a greater change in HR and higher NBAS motor performance cluster score in females. Salivary cortisol, HR change, NBAS cluster scores, and behavioral states after NBAS provided 100% discrimination between male and female infants. These findings suggest that there are neonatal sex differences in behavioral and physiological stress reactivity prior to socialization.