The novel diuretic ethyl (Z)-(3-methyl-4-oxo-5-piperidino-thiazolidin-2-ylidene)acetate (etozolin, Gö 687, Elkapin) has been investigated on 10 volunteers with intact renal function. Administration of 800 mg of the compound caused a significant increase of the renal elimination of water, sodium, chloride magnesium as well as of the sum of the osmotic constituents while an influence on the creatinine-clearance could not be demonstrated. Potassium elimination was also raised, however, at the same time the sodium-potassium ration increased strongly. Due to its long-lasting action etozolin seems particularly indicated for the treatment of cardiac and renal edema as well as for the treatment of hypertension.