A proportion of preterm infants respond to an external airway obstruction by becoming apneic. We have studied 23 infants (median birthweight, 1.14 kg; gestation, 29 weeks) on 80 occasions, to determine the time course of the response and its relationship with spontaneous apnea occurrence. Upper airway flow was measured with a face mask and pneumotachograph, and a tap was turned intermittently to produce an occlusion. A total of 380 occlusions were analyzed. The infants became apneic during the obstruction on 72 occasions (19%), and after the obstruction on 122 occasions (32%). Both of these events were significantly more common than immediately prior to the obstruction, when apnea occurred on 29 occasions (8%). Of the apneas following occlusion 57% were central in type. The point in the respiratory cycle at which obstruction occurred had no effect on the production of apnea. An obstruction score was calculated for each study. This was the mean of the number of apneas during and after each obstruction, expressed as a percentage of the number of obstructions per study. This score was positively correlated with the number of spontaneous apneas recorded. Obstruction score rose from a mean of 20% during days 0-14, to 34% during days 15-28, and thereafter it declined. This pattern may be relevant to the time course of apnea in susceptible infants.