Many aspects of the mechanism of action of cholera toxin (CT), the causative agent of cholera are well established (Fishman, 1990). CT consists of a pentameric B subunit which binds to ganglioside G Ml on the cell surface and an A subunit which activates adenylyl cyclase. The latter process involves the reduction of A to Al peptide which ADP-ribosylates the stimulatory G protein, GS of adenylyl cyclase. There is a distinct lag phase of 15 to 20 minutes between toxin binding and its activation of adenylyl cyclase. Little is known, however, about events during this lag including where Al is generated and how it gains access to GS on the cytoplasmic side of the plasma membrane. In an attempt to identify those steps involved in the intracellular processing of CT and the site of CT-A reduction to the Al peptide, we have employed several known inhibitors of intracellular trafficking to study their effects on the response of human SK-N-MC neurotumor and CaCo-2 intestinal tumor cells to CT. Chloroquine and monensin affect intracellular trafficking by increasing endosomal and lysosomal pH. Monensin is also known to effect glycoprotein processing in the Golgi. Concentrations of chloroquine and monensin as high as 400 μM and 10 μM respectively, had little or no effect on CT stimulation of cyclic AMP in either cell line. Brefeldin A (BFA) however, totally inhibited the response to CT in a time, dose-dependent, and reversible manner with an IC50 of 30 ng mL−1.