We have been studying CFTR channels in guinea pig pancreatic duct cells and rather surprisingly found that luminal HCO3- had a pronounced inhibitory effect on cAMP-activated CFTR chloride currents. The block produced by HCO3- was rapid, voltage-independent and occurred over a physiological range of extracellular HCO3- concentrations. I- and ClO4- were also found to inhibit CFTR currents, but both were less effective than HCO3-. Although we have not identified how HCO3- is able to block CFTR our data suggests that an external anion-binding site on the channel itself is involved. Overall, our results show that luminal HCO3- acts as a potent inhibitor of CFTR channels (and by inference CFTR-mediated secretion), under normal physiological conditions. These data have implications not only for current models of pancreatic duct cell HCO3- transport, but also for other bicarbonate-secreting tissues, such as the liver, GI tract and lungs.