To examine a possible role of indoleamines in the regulation of epithelial sodium absorption, the effect of serotonin (5-hydroxytryptamine) and several derivatives on electrolyte transport was measured in vitro in the baboon bronchus and in the trachea and colon of sodium-deficient rats. Serotonin, melatonin (N-acetyl-5-hydroxytryptamine), and harmaline (1-methyl-7-methoxy-3,4-dihydro-beta-carboline) inhibited sodium transport in all three preparations in a similar manner to the natriuretic agent amiloride. In all three epithelia, sodium absorption via the amiloride-sensitive pathway constitutes a substantial portion of total electrolyte transport, measured as the amiloride-sensitive short-circuit current. Thus 25 microM amiloride inhibited the short-circuit current 21% in the rat trachea, 63% in the baboon bronchus, and 90% in the rat colon. Serotonin, melatonin, and harmaline inhibited the amiloride-sensitive portion of the short-circuit current from the luminal side of the epithelium. The inhibition was rapid, requiring only seconds, and maximal inhibition by serotonin was identical to that by amiloride. When sodium was omitted from the luminal solution, the short-circuit current was reduced a similar amount, suggesting that sodium absorption was being inhibited by both amiloride and the indoles. The IC50 value for amiloride was 50 nM in the baboon bronchus and 500 nM in the rat colon. In contrast, the IC50 value for serotonin was 0.4 mM in the baboon bronchus and 8 mM in the rat colon. These results, together with the wide distribution of amine-precursor-uptake-and-decarboxylation (APUD) cells in the respiratory and intestinal tract, suggest that certain indoleamines could play a role as local regulators of fluid and electrolyte transport. For example, in the airways, indoleamines may be one of the factors involved in regulation of the depth of the periciliary fluid layer.