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1. The effects of low amiloride concentrations (less than 2.5 x 10(-5) M) on the apical membrane (Vm) and transepithelial (Vms) potential differences, transepithelial resistance (Rep) and apical/basolateral membrane resistance ratio (Rm/Rs) have been determined in the gall-bladders of the rabbit and guinea-pig. 2. Vm was hyperpolarized, Vms decreased towards negative values, Rm/Rs increased, but Rep remained unchanged. 3. K+ selectivity of the apical membrane was also checked by increasing luminal K+ concentration from 5.9 to 29.4 mM; the corresponding change in the apical electromotive force was much higher in the rabbit than in the guinea-pig. 4. Pre-incubations with Cl--and HCO3--free salines did not modify K+ selectivity in the rabbit, but nearly doubled it in the guinea-pig. 5. Pre-incubations with control solutions containing 10(-5) M-amiloride in the lumen converted the apical membrane of rabbit gall-bladder into a perfectly K+-perm-selective membrane; the same results were obtained in the guinea-pig only on bathing the tissue with Cl-- and HCO3--free salines together with 10(-5) M-amiloride. 6. It is suggested that in the apical membrane of rabbit gall-bladder K+ and Na+ conductances exist, whereas in guinea-pig gall-bladder both K+, Na+ and anion (Cl- and/or HCO3-) conductances are present; in both species conductive Na+ pathways are inhibited by amiloride. .