To find out whether there is any relationship between the ventilatory response to hypoxia and the sudden infant death syndrome (SIDS), the effects of mild induced hypoxia (PIO2 [pressure of O2 in inspired air], 120 mm Hg = 17% O2) in 16 infants aged 2 wk-6 mo. were studied. Eight had recurrent apneic spells (apnea group) (5 had aborted SIDS and 3 had recurrent apnea in the intensive care nursery) and 8 were well preterm infants about to fly in a pressurized airplane (PIO2, 120 mm Hg) (control group). Mean birth weights were 2245 and 1400 gm and mean gestational ages were 35 and 30 wk. Postconceptual ages (41.8 and 41.3 wk) were almost identical. Heart rate was obtained from an ECG, and respiratory rate and pattern were obtained from a pneumogram. In addition, end-tidal PCO2 [partial pressure of CO2] and PN2 [partial pressure of N2] or PO2 [partial pressure of O2] were obtained with a nasal catheter and gas analyzers. In the apnea group with inhalation of 17% O2, an increase in periodic breathing and an increase in both rate and total duration of respiratory pauses was observed. In the control group there were no significant changes. Heart rate and PCO2 did not change in either group. Infants prone to apnea possibly may have unique respiratory responses to mild induced hypoxia.