The expression of the cystic fibrosis (CF) gene on its introduction into nonepithelial somatic cells has recently been shown to result in the appearance of distinctive low conductance chloride channels stimulated by cyclic AMP (Kartner, N., Hanrahan, J.W., Jensen, T.J., Naismith, A.L., Sun, S., Ackerley, C.A., Reyes, E.F., Tsui, L.-C., Rommens, J.M., Bear, C.E., and Riordan, J.R. (1991) Cell 64, 681-691; Anderson, M. P., Rich, D.P., Gregory, R.J., Smith, A.E., and Welsh, M.J. (1991) Science 251, 679-682). Since Xenopus oocytes provide a powerful system for ion channel characterization, we have examined whole cell and single channel currents in them after injection of cRNA to program the synthesis of the cystic fibrosis transmembrane conductance regulator (CFTR). This has enabled the direct demonstration that the cyclic AMP activation is mediated by protein kinase A and that CFTR is without effect on the endogenous calcium-activated chloride channels of the oocyte, which have been well characterized previously and widely used as reporters of the expression of G-protein-coupled receptors. These findings strengthen the argument that the CF gene codes for a novel regulated chloride channel rather than a regulatory protein which can modulate separate chloride channel molecules.