The effect of insulin on short-circuit current and on the sodium transport system of the toad bladder has been examined. The rate coefficients for sodium movements across the mucosal and serosal barriers of the bladder epithelium were studied by observing the approach to a steady value of the flux of Na22 across the bladder. Insulin added to the solutions bathing both surfaces of the bladder causes a marked increase in short-circuit current. A smaller effect may also be elicited by adding the hormone to either the serosal or the mucosal bathing media. Insulin causes an important increase in the rate coefficient for sodium movement from the cells to the serosal solution with no significant change in the rate co-efficients for sodium movement across the mucosal surface of the epithelial cells. The results indicate that the action of insulin is the result of a stimulation of the active transport step at the serosal surface of the cells. Insulin does not appear to modify the permeability to sodium of the mucosal surface.