The effects of ethyl (Z)-(3-methyl-4-oxo-5-piperidino-thiazolidin-2-ylidene) acetate (etozolin, Elkapin) on whole kidney function and on fluid, sodium and potassium reabsorption in proximal convoluted tubules and loops of Henle of superficial nephrons were studied in rats using clearance and micropuncture techniques. Etozolin (50 mg/kg i.v. initial dose; 50 mg/kg per hour i.v.) markedly increased urinary fluid and sodium excretion and decreased glomerular filtration rate. Fractional tubular sodium reabsorption fell by 8.7%. Fluid and electrolyte reabsorption in the proximal convolution was not significantly influenced by etozolin. The substance significantly increased flow rate, sodium and potassium concentration and decreased tubular fluid/plasma inulin ratio of the early distal tubular, indicating an inhibitory effect on fluid and electrolyte reabsorption in the loop of Henle. Although etozolin is chemically different its site of diuretic action is the same as that of furosemide and ethacrynic acid.