In order to verify that the novel uricosuric loop-acting diuretic S-8666 has an additional site of action in the distal tubule, we investigated the additive diuretic effect of S-8666 during furosemide-induced diuresis in rats. Intravenous bolus injection of S-8666 (3-30 mg/kg) and trichlormethiazide (TCM) (1-10 mg/kg) caused dose-dependent increases in Na+ excretion during furosemide-induced diuresis (primary, 10 mg/kg i.v.; sustaining, 10 mg/kg/hr i.v.), whereas injection of furosemide (5 mg/kg) did not. Ca++ excretion was decreased by injection of each drug. A significant reduction in Ca++/Na+ was observed at all doses of S-8666 and TCM. Additional natriuretic and hypocalciuric effects were lower in the S-8666 than in the TCM group. From these observations we conclude that: 1) S-8666 has a natriuretic effect which is additive to that of furosemide; 2) S-8666 attenuates the calciuric effect of furosemide; and, 3) S-8666 may have an additional site of action in the distal tubule.