Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent chloride channel in epithelial cells; recently, we identified it in mast cells. Previous work that we confirmed showed that interferon gamma (IFNgamma) down-regulated CFTR expression in epithelial cells (T84), but by contrast, we found that IFNgamma up-regulated CFTR mRNA and protein expression in rat and human mast cells. IFNgamma up-regulation of CFTR in mast cells was inhibited by p38 and extracellular signal-regulated kinase (ERK) kinase inhibitors but not a Janus tyrosine kinase (JAK)2 inhibitor, whereas in T84 cells IFNgamma-mediated down-regulation of CFTR was JAK2-dependent and ERK- and p38-independent. Furthermore, IFNgamma down-regulation of CFTR in T84 epithelial cells was STAT1-dependent, but up-regulation of CFTR in mast cells was STAT1-independent. Thus, differential regulatory pathways of CFTR expression in mast cells and epithelial cells exist that depend upon either p38/ERK or JAK/STAT pathways, respectively. Surprisingly, IFNgamma treatment of mast cells inhibited Cl(-) efflux, in contrast to up-regulation of CFTR/mRNA and protein expression. However, down-regulation of Cl(-) flux correlated with IFNgamma-mediated inhibition of mediator secretion. This and other work suggests that the effect of IFNgamma on CFTR expression in mast cells is important for their function.