Bumetanide is well known for its ability to inhibit the nonconductive Na⁺-K⁺-2Cl cotransporter. We were surprised in preliminary studies to find that bumetanide in the contraluminal bath also inhibited NaCl absorption in the human sweat duct, which is apparently poor in cotransporter activity. Inhibition was accompanied by a marked decrease in the transepithelial electrical conductance. Because the cystic fibrosis transmembrane conductance regulator (CFTR) Cl channel is richly expressed in the sweat duct, we asked whether bumetanide acts by blocking this anion channel. We found that bumetanide 1) significantly increased whole cell input impedance, 2) hyperpolarized transepithelial and basolateral membrane potentials, 3) depolarized apical membrane potential, 4) increased the ratio of apical-to-basolateral membrane resistance, and 5) decreased transepithelial Cl conductance (G_Cl). These results indicate that bumetanide inhibits CFTR G_Cl in both cell membranes of this epithelium. We excluded bumetanide interference with the protein kinase A phosphorylation activation process by "irreversibly" phosphorylating CFTR [by using adenosine 5'-O-(3-thiotriphosphate) in the presence of a phosphatase inhibition cocktail] before bumetanide application. We then activated CFTR G_Cl by adding 5 mM ATP. Bumetanide in the cytoplasmic bath (10³ M) inhibited ~71% of this ATP-activated CFTR G_Cl, indicating possible direct inhibition of CFTR G_Cl. We conclude that bumetanide inhibits CFTR G_Cl in apical and basolateral membranes independent of phosphorylation. The results also suggest that >10⁵ M bumetanide cannot be used to specifically block the Na⁺-K⁺-2Cl cotransporter.