The electrical properties of epithelial cell membranes in human descending and ascending colon were studied using microelectrodes and the Na channel blocker amiloride. Under control (pre-amiloride) conditions, the transepithelial electrical measurements in the two colonic segments were similar. The mucosal addition of 0.1 mM amiloride to descending colon totally abolished the transepithelial voltage (Vt) and short-circuit current (Isc), and significantly increased the total tissue resistance (Rt) by 19% (p less than 0.005). Intracellular recordings in descending colon obtained with microelectrodes revealed that the transepithelial effects of amiloride reflected hyperpolarization of the apical membrane and a significant increase in apical membrane resistance, changes which were consistent with amiloride-blockade of apical Na channels and complete inhibition of electrogenic Na transport. An appreciable amiloride-insensitive conductance was also present in the apical membrane of the descending colon. In contrast, the mucosal addition of 0.1 mM amiloride to ascending colon decreased Vt by only 30% (p less than 0.02) and Isc by 43% (p less than 0.05), but had no significant effect on Rt. These results indicate that electrogenic Na transport accounts totally for Isc in human descending colon in vitro, but only partly for Isc in human ascending colon, suggesting that Na transport in ascending colon is mediated primarily by electroneutral processes, and the apical membrane of human descending colon contains an amiloride-sensitive conductance for Na in parallel with an amiloride-insensitive conductance, which may transport K.