The first Specific Aim is to determine the ventilatory response to CO2 in the developing rat and to determine age-related changes in the control of breathing. Specific Aim 2 is to test the hypothesis that olfactory carbonic anhydrase activity is present at birth in the rat pup. In regards to SIDS, the investigator postulates that an exogenous stressor, i.e., rise in CO2 by rebreathing in the face-down, prone sleeping position, stimulates the olfactory CO2 chemosensors, which will mediate a ventilatory inhibition prior to the full development of cardiorespiratory homeostasis.