Cystic fibrosis is a major inherited disorder involving abnormalities of fluid and electrolyte transport in a number of different organs. Epithelial cells of cystic fibrosis patients have a decreased capacity to secrete chloride in response to cAMP-mobilizing agents because of the mutation of a single gene. The gene product, the cystic fibrosis transmembrane conductance regulator or CFTR, is a chloride channel. The most frequent mutation is a deletion of phenylalanine in position 508 (delta F508-CFTR) that reduces both the expression of the CFTR protein at the cell surface, and the activity of the Cl- channel. This work presents the properties of NS004, a substituted benzimidazolone, which is the first activator of normal and mutant CFTR-associated chloride channels to be described. NS004 activated CFTR and delta F508-CFTR Cl- channels expressed in Xenopus oocytes, and increased 125I efflux (via the Cl- channel) from Vero cells expressing CFTR and delta F508-CFTR. Application of NS004 to the external side of outside-out patches excised from these CFTR- and delta F508-CFTR-expressing cells induced a marked and reversible increase in channel activity.