Leptin is secreted into the gastric juice by epithelial Chief cells, and reaches the duodenum in a biological intact active form. We have assessed the possibility that this gastric leptin crosses the intestinal mucosa by transcytosis through enterocytes to reach blood circulation. Endogenous gastric leptin secretion was triggered by cholinergic stimulation. In another set of ecperiments, recombinant leptin was inserted in vivo into the duodenal lumen. Plasma levels of leptin were assessed by Enzyme-ImmunoAssay and western blot, and duodenal tissue was processed for immunocytochemistry. We first observed that leptin was found inside duodenal enterocytes from fed rats but not from fasted ones. Stimulation of gastric secretion by a cholinergic agent led to rapid increases in plasma leptin levels (202 ± 39%) except when the pylorus was clamped. Insertion of recombinant leptin into the duodenal lumen raised plasma leptin concentrations (558 ± 34%) quite rapidly while carrier solution without leptin had no effect. The use of fluorescein isothiocyanate-tagged leptin reinforced these results. Light and electron microscopy revealed the cellular compartments involved in its transcytosis, namely the enterocyte microvilli, the endocytotic vesicles, the Golgi complex and the baso-lateral interdigitations. Leptin was also present in the lamina propria, in capillary endothelial cell plasmalemmal vesicles and in capillary lumina. These results demonstrate that gastric exocrine leptin is internalized by duodenal enterocytes and delivered to the lamina propria and blood circulation. It may thus be able to play important paracrine and endocrine functions for the control of gastric emptying and nutrient absorption.