Many epithelial organs, both absorptive, like the intestine and gall bladder, and secretory, like the pancreas and salivary glands, transport salt and water in isotonic proportions, i.e. in the absence of a substantial trans-epithelial osmotic gradient. It seems reasonable to suggest that the basic mechanisms involved in these transport processes are similar in all such organs. To examine this hypothesis, we have studied the secretion of pancreatic juice by the ducts of the perfused cat pancreas, and the secretion of saliva by the perfused rabbit mandibular gland. We have compared the effects of removing one or more perfusate constituents, or adding pharmacological blocking agents, or both. These studies suggest that a Na⁺/Cl^- carrier and a Na⁺/H⁺ carrier (or their equivalent) are responsible for secretion in both glands. In the mandibular gland, the carriers can act independently (i.e. Cl^- and HCO₃^- can each sustain secretion): in the pancreatic ducts they appear to be operationally linked (i.e. Cl^- cannot sustain secretion in the absence of HCO₃^-. What this means in molecular terms remains unknown.