A gastric tube with esophageal balloon was designed for neonatal use. The response of the balloon was evaluated. In vitro studies involved comparisons of pressure changes (delta P) in a flask, as measured directly, or as transferred via the balloon. Values for delta P ranged frm 5 cmH2O to 40 cmH2O, and frequencies from 10 to 1620 cycles per minute. The volume of air within the balloon was varied from 0.1 to 0.4 ml. At each combination of pressure, frequency and balloon volume, the balloon transmitted an accurate, if minimally lower delta P, than that imposed. In a rabbit, intraesophageal delta Ps measured via the balloon were compared with intrapleural delta Ps, measured via a chest tube. The frequency rates and delta Ps achieved were similar to those encountered in the clinical situation. The delta s measured via the balloon were frequently equal to or sometimes minimally lower than the actual pleural pressure changes. The prototype meets recommended dimensions for esophageal balloon use in the neonate and allows for accurate routine measurements for calculation of pulmonary function. The device is now available commercially.