Ca2+ channel blocker (sensit) and calmodulin antagonists (thioridazine, perphenazine, oxyprothepine) applied to the mucosal side of frog urinary bladder, weakened the response of epithelial cells to vasopressin. Thioridazine (2.7 X 10(-5) mol X l-1) and sensit (1.7 X 10(-4) mol X l-1) applied to the serosal side rapidly increased the permeability of the epithelia for sodium and potassium ions along the concentration gradient (from serosa to mucosa). The same concentrations of these blockers when applied to the mucosal side of frog urinary bladder selectively decreased vasopressin stimulated water permeability and did not influence ionic permeability. Both thioridazine and sensit decreased the short-circuit current across frog skin. The results show that the Ca2+ channel blocker and the calmodulin antagonists tested influenced water and ionic transport across the epithelial cell membranes, and had different effects upon the apical and the basolateral cell membranes.