Background and Aims: The cystic fibrosis transmembrane conductance regulator (CFTR) is an important pathway for duodenal mucosal bicarbonate secretion. Duodenal biopsies from cystic fibrosis patients secrete bicarbonate in response to heat-stable enterotoxin from Escherichia coli (STa), but not cAMP. To explore the mechanism of STa-induced bicarbonate secretion in cystic fibrosis more fully, we examined the role of CFTR in STa-stimulated duodenal bicarbonate secretion in mice. Methods: In vivo, the duodenum of CFTR ((-/-); CF) and control mice was perfused with forskolin (10^-4 mol/L), STa (10^-7 mol/L), uroguanylin (10^-7 mol/L), 8-Br-cGMP (10^-3 mol/L), genistein (10^-6 mol/L) plus STa, or herbimycin A (10^-6 mol/L) plus STa. In vitro, duodenal mucosae were voltage-clamped in Ussing chambers and bicarbonate secretion was measured by pH-stat. The effect of genistein, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS, 10^-4 mol/L) and chloride removal was also studied in vitro. Results: Control, but not CF mice produced a significant increase in duodenal bicarbonate secretion following perfusion with forskolin, uroguanylin, or 8-Br-cGMP. However, both control and CF animals responded to STa with significant increases in bicarbonate output. Genistein and herbimycin A abolished this response in CF mice, but not in controls. In vitro, STa-stimulated bicarbonate secretion in CF tissues was inhibited by genistein, DIDS, and chloride-free conditions, while bicarbonate secretion persisted in control mice. Conclusions: In the CF duodenum, STa can stimulate bicarbonate secretion via tyrosine kinase activity resulting in apical chloride/bicarbonate exchange. Further studies elucidating the intracellular mechanisms responsible for such non-CFTR mediated bicarbonate secretion may lead to important therapies for cystic fibrosis.