We have found that excess irregularity of ventilatory frequency, alveolar hypoventilation and abnormal response to CO2 breathing are features of some infants who die of SIDS. From approximately 60 infants referred per year for evaluation of near-death episodes, we will select those with demonstrated abnormal ventilatory response to CO2 breathing (aborted SIDS) and follow them for 2 years along with a cohort of normal infants. At similar ages following conception, at times of stress (infection and immunization) studies will be repeated. In order to determine the pathophysiology of the abnormal CO2 response, we will also investigate the activity of other respiratory control reflexes. We will study ventilatory chemoreceptor function in the parents of infants with aborted SIDS and of controls in order to determine if the level of responsiveness in infants might have a hereditary basis.