A series of sulphonylthioureas related to torasemide, a high ceiling loop diuretic, were synthesized and found to inhibit the Na+ 2Cl- K+ co-transporter of the thick ascending limb of the loop of Henlé. Their diuretic properties were studied (30 mg kg-1) after oral administration to rats. Lipophilic derivatives, very active in-vitro, were found inactive orally and intraperitoneally in rats. The four most active compounds were examined for their dose-dependent diuresis. Three of them showed a potency, water and electrolyte excretion similar to torasemide. The fourth molecule, a sulphonylthiourea (BM 20), exhibited relative potassium-sparing properties and a minimal diuretic dose of 0.001 mg kg-1, 200 times lower than torasemide.