Cystic fibrosis (CF) is a monogenic autosomal recessive disease caused by mutation in the cystic fibrosis gene that encodes the cystic fibrosis transmembrane conductance regulator (CFTR), which is an ion-channel that conducts negatively charged chloride ions. Cystic fibrosis related diabetes (CFRD) is the leading complication of CF and exocrine pancreatic dysfunction affects ~85% of patients. Recently, it was discovered that rat a-cells and b-cells in the islets of Langerhans express both CFTR mRNA and protein. The aim of this study was to investigate the presence of active CFTR channels in pancreatic b-cells and if these influence insulin secretion. For this purpose we have used the patch-clamp technique and capacitance measurements on single mouse b-cells and insulin secretion measurements using RIA. First we measured the presence of CFTR in b-cells using the patch-clamp technique. A membrane conductance of 0.05 ± 0.01 nS/pF (n=10) and 1.05 ± 0.22 nS/pF at negative and positive potentials, respectively, was activated by the cAMP-increasing agent forskolin. The conductance was significantly reduced (P<0.001) and the current almost totally inhibited in the presence of 10 microM of the CFTR-antagonist, CFTRinh-172. Glucose-stimulated and cAMP-amplified insulin secretion measured on islets was not reduced using this concentration although there was a tendency towards reduction. Moreover, exocytosis elicited by a train of ten membrane depolarisations and measured as an increase in membrane capacitance on single b-cells was significantly reduced by 70 ± 10% (P<0.05, n=6) in the presence of 10 microM CFTRinh-172. We conclude that active CFTR is present in mouse pancreatic b-cells and that it has a crucial role in exocytosis of secretory granules in the b-cells.