We have performed studies on growing puppies to investigate how the newborn and developing animal responds to stress such as hypoxia during sleep. We have been interested in the cardiovascular and ventilatory responses to hypoxia. Hence we have been recording and analyzing ventilation, heart rate, heart rate variability and blood pressure during normoxia and hypoxia during REM and quiet sleep. We have found that the young responds differently from the older dog and this differences is sleep state dependent. The young dog depresses minute ventilation and O2 consumption in response to hypoxia in quiet sleep but stimulates its ventiliation concomitantly with metabolic depression in REM sleep. In the older puppy, hypoxia stimulates minute ventilation with no change in O2 consumption in both sleep states. In addition, the sensitivity of the carotid baroreflex is enhanced as puppies mature. We believe that these studies constitute a first basic step in the understanding of the control of breathing in the young animal and have potentially very important implications for SIDS and premature infants.