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1. Brush border membrane (Vm) and transepithelial (Vms) potential differences, transepithelial resistance (Rep) and the brush border membrane/basolateral membrane resistance ratio (Rm/Rs) have been measured in distal colons taken from new-born and 4-day-old pigs. 2. Vm remained independent of the age of the animal. Substituting choline or Tris for Na+ hyperpolarized Vm; substituting SO4(2-) for Cl- or increasing the concentration of K+ depolarized Vm in colons taken from new-born pigs. K+ depolarization was not seen using colons taken from 4-day-old animals. 3. The effect of Na+ or Cl- replacement on Vms and Rm/Rs was in agreement with the hypothesis that the brush border membrane of the distal colon possessed a conductance for both these ions. The brush border membrane of the new-born, but not the 4-day-old pig colon possessed an additional conductance for K+. 4. These electrical measurements were repeated using 4-day-old pig distal colons in the presence of amiloride, to enable values for the electromotive force across the brush border membrane (Em) and the basolateral membrane (Es) to be calculated. The Em of the 4-day-old pig colon was found to be about 23 mV negative with respect to the mucosal solution. Cl- conductance appears to predominate in colons taken from these very young animals. 5. Separate experiments showed net transport of Na+ and measured short-circuit current to be partly dependent upon the presence of Cl. Na+ transport measured in the absence of Cl- was approximately equal to that calculated from the measurement of short-circuit current. 6. It is suggested that the brush border membrane of the neonatal pig colon is conductive to both Na+ and Cl-, that the transport of these ions are partly coupled in some way and that this alone is sufficient to explain the discrepancies noted previously when measuring Na+ flux and short-circuit current in Cl- -containing medium. The possible physiological importance of this transient transport function is discussed.