Resting ventilation and ventilatory responses to 100% O2 and to 5% CO2 in air were measured in REM [rapid eye movement] and non-REM sleep in post-neonatal infants. Normal controls were compared to infants with prolonged apnea and to siblings of sudden infant death victims. No significant differences in ventilatory responses were found between the groups. Apnea may occur in infants whose central and peripheral chemoreceptor activity is normal while they are breathing.