Central respiratory drive was studied in 13 piglets of both sexes varying in age from 19 to 67 days. The distal trachea was cannulated and the maximum rate of isometric inspiratory pressure change (dP/dt)(max), was measured at the airway. Curves were constructed relating this measurement to changes in arterial PCO(2) during carbon dioxide rebreathing. Data were obtained at intervals corresponding to stepwise reductions in central respiratory drive produced by added chloralose anaesthesia. Laryngeal reflex activation was achieved by electrical stimulation of the superior laryngeal nerves (SLN). This caused permanent respiratory arrest at a critical level of central respiratory depression expressed as the slope of the curve relating (dP/dt)(max) to arterial PCO(2). Severely anemic piglets showed markedly decreased central respiratory drive at a given dose of anesthesia compared to controls. This was consistent with the observed greater sensitivity to laryngeal nerve stimulation in these animals. It is concluded that anemia may be associated with impaired functional maturation of central respiratory mechanisms and consequent susceptibility to laryngeal reflex apnea and asphyxial death. These observations may pertain to factors associated with the sudden infant death syndrome.