Mast cell activation requires Cl(-) flux, which maintains the driving force for entry of extracellular calcium and initiates release of mediators such as histamine. However, chloride channel expression in mast cells has been poorly understood. For the first time, reverse transcriptase-polymerase chain reaction shows that rat-cultured mast cells (RCMC) and peritoneal mast cells (PMC) contain mRNA for the cystic fibrosis transmembrane conductance regulator (CFTR), an important chloride channel. Immunostaining with an anti-CFTR antibody indicates expression of CFTR in PMC and RCMC. Mast cell CFTR is a functional Cl(-) channel because it is capable of mediating Cl(-) flux in response to elevated cAMP. An inhibitor of CFTR-dependent Cl(-) flux, diphenylamine-2-carboxylate down-regulates mast cell mediator release. These results show that rat mast cells express a functional CFTR, which might be important in mediator release.