A number of physiologic phenomena during some apneic/cyanotic episodes in infants and young children cannot be explained sufficiently on the basis of a cessation or reduction in alveolar ventilation alone. These include (1) the extremely rapid development of hypoxemia during some episodes, (2) the occurrence of hypoxemic episodes despite continuous ventilation, (3) differences in the speed of desaturation between different forms of apneic episodes, (4) the presence of continued breathing efforts and yet absent airflow despite bypass of the upper airway, and (5) evidence that apnea and hypoxemia may begin simultaneously. Potential mechanisms to explain these phenomena include the sudden development of an unevenness in the matching between ventilation and lung perfusion, the flow of blood through unventilated areas of lung, and a sudden impairment in gas diffusion. Evidence from physiological and morphological studies suggests that all of these mechanisms, although to a different extent, may be involved in apneic/cyanotic episodes. The recognition that nonapneic mechanisms can cause sudden profound hypoxemia may alter our approach to the identification, treatment, and prevention of these episodes.