The ability to switch from nasal to oral breathing in response to nasal obstruction is crucial for survival, and has been suggested to be an important mechanism in preventing sudden infant death syndrome (SIDS). To know whether the ability to switch from nasal to oral breathing is uniformly present during the early neonatal period, we examined the effects of slow and fast nasal occlusions on the establishment of oral breathing in preterm infants. Slow occlusions were used to mimic more closely occlusions occurring spontaneously. We studied 17 healthy preterm infants [birth weight, 1830 +/- 27 g (mean +/- SE); study weight, 1800 +/- 109 g; gestational age, 32 +/- 1 weeks; postnatal age, 12 +/- 2 days]. We used a nosepiece with a nasal occluder and a flow-through system to measure ventilation. A CO2 sampling catheter at the mouth was used to detect oral breathing. Of 58 occlusions, 29 were slow [resistance increasing slowly from 0 to infinite (occlusion)], and 29 were fast (infinite elastance applied in < 1 sec). Oral breathing was always established following slow and fast occlusions. In 44% of the slow occlusions, oral breathing started before complete occlusion. Arousal was observed in 12/58 (17%) of all occlusions, occurring primarily after initiation of oral breathing. Oxygen saturation and respiratory rate decreased significantly following occlusions, from 96 +/- 0.6 to 87 +/- 1.2% and 49 +/- 2.8 to 38 +/- 2 breaths/min, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)