Five-day-old infants' responses to stress while being heelstuck were examined. Behavioral responses as well as the adrenocortical responses of cortisol release were used as measures of manifest and internal stress response. We divided 67 infants into two groups-those who presented no sounds during the "heelstick" (C group) and those to whom heartbeat sounds were presented during the "heelstick" (E group). Heartbeat sounds were presented to infants at 85 dB. The C group showed more reactive adrenocortisol release and more behavioral responses than the E group. We found the calming effect of heartbeat sounds on stress in newborn infants.