As respiratory difficulty may accompany nipple feeding in preterm neonates, we studied the effect of oral feeding on ventilation in 23 preterm infants. The infants composed two groups based on their postconceptional age at the time of study: Group A comprised 12 infants 34 to 35.9 weeks of age, and group B, 11 infants 36 to 38 weeks. Ventilation was measured via a nasal mask pneumotachometer, and sucking pressure via a nipple that also permitted milk delivery; transcutaneous PO2 and PCO2 were continuously monitored. The feeding pattern comprised an initial period of continuous sucking of at least 30 seconds, followed by intermittent sucking bursts for the remainder of the feed. When compared with an initial semi-upright control period, minute ventilation (V1) during continuous sucking fell by 52 +/- 6% (P less than 0.001) and 40 +/- 2% (P less than 0.001) in groups A and B, respectively. This was the result of a decrease in respiratory frequency and tidal volume and was associated with a fall in TcPO2 of 13 +/- 4 mm Hg (P less than 0.01) in group A and 10 +/- 2 mm Hg (P less than 0.01) in group B. During intermittent sucking, V1 and TcPO2 recovered partially only in the more mature infants (group B). At the end of the feed, TcPCO2 have risen by 3 +/- 1 mm Hg (P less than 0.001) in group A and by 2 +/- 2 mm Hg (P less than 0.05) in group B. Thus oral feeding results in an impairment of ventilation during continuous sucking and the subsequent recovery during intermittent sucking is dependent on postconceptional age.