Summary. In this study the relative ionic permeabilities of the cell membranes of Necturus gallbladder epithelium have been determined by means of simultaneous measurement of transmural and transmucosal membrane potential differences (PD) and by ionic substitution experiments with sodium, potassium and chloride ions. It is shown that the mucosal membrane is permeable to sodium and to potassium ions. The baso-lateral membrane PD is only sensitive to potassium ions. In both membranes chloride conductance is negligible or absent. The ratio of the resistances of the mucosal and baso-lateral membranes, RM/Rs, increases upon reducing the sodium concentration in the mucosal solution. The same ratio decreases when sodium is replaced by potassium which implies a greater potassium than sodium conductance in the mucosal membrane. The relative permeability of the shunt for potassium, sodium and chloride ions is: PK/PNa/Pcl = 1.81: 1.00:0.32.From the results obtained in this study a value for the PK/PNa ratio of the mucosal membrane could be evaluated. This ratio is 2.7. From the same data the magnitude of the electromotive forces generated across the cell membranes could be calculated. The EMF's are -15 mV across the mucosal membrane and -81 mV across the baso-lateral one. Due to the presence of the low resistance shunt the transmucosal membrane PD is -53.2 mV (cell inside negative) and the transmural PD is + 2.6 mV (serosal side positive). The change in potential profile brought about by the low resistance shunt favors passive entry of Na ions into the cell across the mucosal membrane. Calculations show that this passive Na influx is maximally 64 ~o of the net Na flux estimated from fluid transport measurements. The Cl-conductance of the baso-latera| membrane is too small to allow electrogenic coupling of C1 with Na transport across this membrane. Experiments with rabbit gallbladder epithelium indicate that the membrane properties in this tissue are qualitatively similar to those of Necturus gallbladder epithelium.Epithelial tissues involved in transmural Na transport have been divided into "tight" and "leaky" epithelia (Fr6mter & Diamond, 1972).Tight epithelia, such as amphibian skin and urinary bladder, are characterized by high transmural resistances and potential differences (PD).From ionic substitution experiments it was concluded that the transmural *Current address:

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Summary. The permeability of rabbit gallbladder to hydrophilic nonelectrolytes, with molecular weights from 20 to 60,000, has]been studied. Restriction in the diffusion of the small electrolytes is very significant up to glycerol, which suggests permeation through aqueous pores with equivalent radii of 4/~. An extracellular pathway is responsible for the permeation of the larger solutes. This extracellular pathway shows no restriction in diffusion of molecules up to the size of inulin. Dextran (15,000 to 17,000 mol wt) is significantly restricted. Albumin permeability is < 10 .8 cm sec -1. These observations can be equated with equivalent pore radii of ~40 • for the shunt pathway.Increasing osmolarities of the incubation medium cause decreased cell-membrane permeability and increased shunt permeability. 0.5 mM phloretin induces a 60 % reduction in urea permeability and a 168 % increase in antipyrine permeability. No effect on the osmotic water permeability or on the shunt permeability is observed in the presence of phloretin. The apparent activation energy of urea permeation changes from values consistent with diffusion in bulk water, to values consistent with diffusion through hydrocarbon regions. This suggests that the polar route for urea permeation is blocked by phloretin.The contribution of the shunt pathway to osmotic flow induced by sucrose or NaC1 gradients is smaller than 16% according to Poiseuille's flow calculations. Tetraethylammoniumchloride and albumin have been shown to be osmotically more effective than sucrose, suggesting a greater shunt contribution to the total water flow.Extensive studies on nonelectrolyte permeability of rabbit gallbladder epithelium have led to the postulation of small aqueous pores in the cell membranes and a small number of larger pores somewhere in the epithelium [13,48,56]. In this respect gallbladder epithelium resembles the epithelium of kidney proximal tubules [5,23], stomach [1] and small intestine [33]. In all these tissues the anomalous permeability of relatively large hydrophilic solutes, such as sucrose, inulin and even bigger molecules, is explained by postulating the existence of some large aqueous pores in the epithelium.

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