The short-term stability of measures of respiratory sinus arrhythmia (RSA), heart period, and salivary cortisol and the relation of these variables to one another and to measures of social competence in kindergartners were examined. Physiological measures were collected during multiple individual sessions with 30 kindergarten children, ages 5 to 6 years. Three sessions were scheduled at 2-week intervals and included a negative affect task as a challenge. Cardiovascular and cortisol responses to the challenge were assessed. Results indicated that baseline measures of RSA and heart period were stable across time. Baseline cortisol levels were not stable. The change in RSA in response to a task that elicited emotional responses was not stable over time. Consistent with findings observed in adults and older children, changes in RSA were related to the cortisol response. Children who increased cortisol over the course of Session 1 exhibited larger decreases in RSA than children who did not show increased cortisol levels. In addition, the children who had larger decreases in RSA had higher ratings of sociability on the Colorado Childhood Temperament Inventory. Greater decreases in heart period during the task were associated with lower ratings of shyness. The relation of RSA measures to ratings of social competence supports the Polyvagal Theory (Porges, 1995, 2001), which suggests that the ability to engage and disengage with objects and people in one's environment, necessary for social competence, is related to the regulation of vagal control of the heart.