The acute effects of nasogastric (NG) and orogastric (OG) tube placement on pulmonary function of neonates was assessed as a function of infant weight. Lung function was obtained on 14 healthy infants weighing less than 2 kg and 10 infants heavier than 2 kg with an NG and an OG tube in place. Additionally, 15 infants were studied for a third time without gastric intubation. Lung function was determined with an esophageal balloon and by pneumotachography (PeDS) via the least mean square analysis technique. Neither the below-2 kg infants nor the above-2 kg infants had apparent clinical compromise with NG and OG tube placement. Infants weighing less than 2 kg, however, demonstrated diminished minute ventilation and respiratory rate and had increased pulmonary resistance, resistive work of breathing, and peak transpulmonary pressure change with NG tube, as compared to OG tube, placement. The above-2 kg infants demonstrated no change in pulmonary function with NG vs. OG tube placement. These data indicate that small neonates demonstrate significant pulmonary compromise with NG placement that may not be clinically apparent.