Protamine, a naturally occurring arginine-rich polycationic protein (pI 9.7 to 12), was tested in Necturus gallbladder using a transepithelial AC-impedance technique. Protamine sulfate or hydrochloride (100 micrograms/ml = 20 microM), dissolved in the mucosal bath, increased transepithelial resistance by 89% without affecting the resistance of subepithelial layers. At the same time, transepithelial voltage (psi ms) turned from slightly mucosa-positive values to mucosa-negative values of approximately +1 to -5 mV. The effect of protamine on transepithelial resistance was minimal at concentrations below 5 micrograms/ml but a maximum response was achieved between 10 and 20 micrograms/ml. Resistance started to increase within 1 min and was maximal after 10 min. These effects were not inhibited by serosal ouabain (5 X 10(-4) M) but could be readily reversed by mucosal heparin. The sequence of protamine effect and heparin reversal could be repeated several times in the same gallbladder. Mucosal heparin, a strong negatively charged mucopolysaccharide, or serosal protamine were without effect. Mucosal protamine reversibly decreased the partial ionic conductance of K and Na by a factor of 3, but did not affect Cl conductance. Net water transport from mucosa to serosa was reversibly increased by 60% by protamine. We conclude that protamine reversibly decreases the conductance of the cation-selective pathway through the tight junction. Although this effect is similar to that reported for 2,4,6-triamino-pyrimidinium (TAP), the mechanism of action may differ. We propose that protamine binds to the apical cell membrane and induces a series of intracellular events which leads to a conformational alteration of the tight junction structure resulting in decreased cationic permeability.