Duodenum is thought to go through a solid-core stage followed by recanalization during its development. This study investigates the role of apoptosis in normal duodenal development, especially during widening of the lumen, and hence, the possible role of apoptosis in duodenal atresia (DA). Twenty-four time-mated Sprague-Dawley rats were killed from day 13 to day 20 of gestation. Duodenums of 3 fetuses were chosen randomly from each rat and processed. Apoptosis was determined by the terminal deoxytransferase-mediated biotin dUTP nick-end labeling (TUNEL) technique (ApopTag). Apoptosis count and cross-sectional areas were measured with an image analyzer (MetaMorph). The number of apoptotic cells per unit area duodenum peaked on day 15 for the mucosal/submucosal layer and on day 14 for the muscular/mesenchymal layer. The maximal number of apoptotic cells per cross-section of duodenum was between 7 and 8. The cross-sectional areas of the duodenal wall and lumen increased exponentially between day 17 and day 19 while duodenal-wall thickness remained relatively constant throughout duodenal development. The localization, timing, and intensity of apoptosis do not suggest that apoptosis is responsible for the widening of the duodenal lumen; enlargement of the lumen is related to the increase in duodenal circumference. Apoptosis thus may not be involved in the pathogenesis of DA.