In 32 anaesthetized piglets 1 to 42 days of age the distal trachea was cannulated and pressure changes were recorded. The proximal trachea was cannulated for introduction of test fluids into the laryngeal area. Arterial pressure, heart rate, and central venous pressure were continuously recorded. Arterial blood samples were obtained at intervals and analyzed for PO2, PCO2, pH, and hematocrit. Normal saline produced no, or brief, transitory alterations of the respiratory pattern and arterial pressure. In contrast, instillation of distilled water produced apnea in 29 of 30 piglets. In 20 the apnea was sustained. Eleven died within approximately 50 minutes of asphyxia. An additional nine were expected to die with continuing apnea (PO2, 10 to 15 mm Hg; PCO2 greater than 100 mm Hg; pH smaller than 6.8) but the sequence was interrupted by replacement of water with saline. Twenty-three of 29 piglets showed an apneic response to cow's milk similar to that seen with distilled water. Seven died of asphyxia and an additional three showed sustained respiratory inhibition until milk was replaced with saline. The responses were completely abolished by superior laryngeal nerve (SLN) sectioning. Electrical stimulation of the SLN produced sustained apnea in seven of eight piglets tested. Studies in two 3-day-old lambs showed similar discrimination but only transitory apnea with water or cow's milk. Both died during SLN stimulation. Two ewes showed insignificant responses. These findings suggest a lethal reflex mechanism with implications for the SIDS problem.