While in a state of quiet sleep, 15 full-term male, 15 full-term female, and 15 premature male infants were presented a single auditory stimulus. Heart rate was continuously monitored and scored for mean and standard deviation in the 10-sec prestimulus period, as well as for peak and trough magnitude of the response and the latencies to these points. The infants tended to show a diphasic response, acceleration followed by rebound deceleration below baseline. Prestimulus mean and variability were inversely related, and significantly predictive of the amount of acceleration. Premature infants showed a high resting heart rate, and a milder response to the stimulus. Also evident were larger accelerations and longer latencies to trough for full-term females.