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Hôpital des Enfants Malades, INSERM U 30 (Tour Technique), 149, rue de Sèvres, 75730 PARIS Cedex 15, France

1. A study has been made of the effects of anionic substitutions on the electrical potential difference (p.d.) and conductance characteristics of the antiluminal (peritubular) membrane of the proximal tubule of Necturus kidney. The tubular lumina were filled with oil in order to minimize potential and conductance contributions from luminal membrane and from paracellular shunt pathway.

2. Isosmotic substitutions, [A]_o for [Cl]_o, produced the following average changes in membrane p.d. (mV): F^- +1·7, BrO₃^- +0·1, Br^- -4·5, ClO₃^- -5·2, I^- -7·9, NO₃^- -12·1, ClO₄^- -17·8, SCN^- -25·3.

3. The amplitude of the depolarization caused by increase in K concentration (K-depolarization) in the peritubular perfusate was found to increase during perfusion of the tissue with ClO₄^- (by 78%), SCN^- (45%), I^- (23%), NO₃^- (20%), Br^- (16%); it decreased with F^- (by 17%).

4. Comparison of membrane p.d. at peak K-depolarization in the control state (during KCl perfusion) with that obtained in the experimental state (during KA perfusion) was found to be more reliable than determination of bi-ionic potentials as a qualitative estimate of the permeabilities of the various anions (P_A) relative to that of chloride (P_Cl).

5. Study of both peak K-depolarization p.d. and bi-ionic potentials yielded the following sequence for halide anion permeabilities: P_F > P_Cl > P_Br > P_I. The peritubular membrane was found to be substantially more permeable to NO₃^-, ClO₄^- and SCN^- than to Cl^-.

6. The sequence of membrane conductances during anionic substitutions was Cl^- BrO₃^- < Br^- ClO₃^- < I^- F^- < NO₃^- < ClO₄^- < SCN^-.

7. From the changes in p.d. induced by K-depolarization, the absolute values of p.d. at peak K-depolarization and from the changes in membrane conductance induced by anionic substitutions, it may be inferred that Br^-, I^-, NO₃^-, ClO₄^- and SCN^- all increase P_K; and that F^- increases P_Na (though a smaller increase in P_K cannot be excluded).