Recent in vitro studies in human colon have demonstrated marked segmental differences in electrogenic Na transport. In the present study, the Na channel blocker amiloride was used further to characterise basal and aldosterone-induced electrogenic Na transport in isolated human distal and proximal colon. Bathed in NaCl Ringer solution, distal and proximal colon exhibited similar basal electrical properties, but the amiloride-sensitive short-circuit current (Isc) was 200% greater in the distal than in the proximal segment. Bathed in choline-Cl Ringer solution, total Isc decreased by 97% in distal colon and by 88% in proximal colon, indicating that Na dependent transport process(es) account almost entirely for the Isc in both segments. Substituting Na2SO4 for NaCl Ringer solution (i) increased amiloride-sensitive Isc by 56% (p less than 0.01) in distal colon but had no effect on amiloride-sensitive Isc in proximal colon, and (ii) decreased amiloride-insensitive Isc in distal and proximal colon by 52% (p less than 0.05) and 81% (p less than 0.001) respectively. After the addition of nystatin to the apical membrane, the relationship between total Isc and mucosal Na concentration indicated that the activity of the basolateral membrane Na pump was similar in both colonic segments. In a further series of experiments, exposure of distal colon to 1 mumol/l aldosterone for 5 h increased total Isc by 52% (p less than 0.05), which reflected stimulation of its amiloride-sensitive component; in contrast, aldosterone had no effect on proximal colon.(ABSTRACT TRUNCATED AT 250 WORDS)