In contrast with cicletanine, its urinary sulfoconjugate metabolite (cicletanine sulfate) was active on membrane ion transport in human red blood cells. Cicletanine sulfate was a more potent inhibitor of the Na+ dependent [Cl-/HCO3-] exchanger (IC50 = 9 +/- 3 x 10(-5) mol/l; mean +/- SD of 4 experiments) than cicletanine (IC50 = 10(-3) mol/l). This inhibitory potency was intermediate between that of xipamide (IC50 = 2 x 10(-5) mol/l) and that of furosemide (IC50 = 2 x 10(-4) mol/l). Moreover, cicletanine sulfate exhibited modest inhibitory potency against the [Na+,K+,Cl-]-cotransport system (IC50 = 1 +/- 0.3 x 10(-3) mol/l; mean +/- SD of 4 experiments) and poor inhibitory activity against the [K+,Cl-]-cotransport system. Cicletanine sulfate was unable to modify the activity of Cl(-)-independent membrane carriers (Na+:H+ exchanger, Ca2+ pump, Na+:Li+ countertransport system and Na+,K+ pump). Following renal intraarterial administration in rats, cicletanine sulfate and not cicletanine, exhibited salidiuretic activity. In conclusion, the urinary sulfo-conjugate of cicletanine is an active anion transport inhibitor and natriuretic metabolite. In fact, this metabolite may be responsible for the salidiuretic action of cicletanine.