OBJECTIVE We studied the maturation of esophageal body and upper esophageal sphincter (UES) motor responses that protect against reflux in newborns. METHODS The responses of esophageal body and UES on midesophageal provocation were quantified using a micromanometric water perfusion system and a specially designed catheter with a UES sleeve and 5 side-hole recording sites. Eighteen healthy premature infants were studied twice, at 33.4 weeks' and 35.7 weeks' mean postmenstrual ages (PMAs). RESULTS Mean threshold volumes and the distribution of responses including secondary peristalsis, UES pressure increase, and primary peristalsis were similar for air and liquids. Secondary peristalsis and UES pressure increases were volume dependent for both studies, but their characteristics changed as PMA increased in that (1) completely propagated secondary peristalsis with liquids increased; (2) proximal esophageal contraction duration was shorter for air and liquids; and (3) propagating velocity for liquids was faster. CONCLUSIONS Esophageal and UES motor responses to abrupt midesophageal provocation are present as early as 33 weeks' PMA, and the response-characteristics improve during development. Reflexes that may facilitate esophageal clearance include (1) secondary peristalsis; (2) esophago-UES-contractile reflex; and (3) esophago-deglutition response.