The novel diuretic (Z)-(3-methyl-4-oxo-5-piperidino-thiazolidin-2-ylidene)-acetic acid ethylester (Etozolin; Elkapin) has been investigated against a benzothiazide (Vs) and placebo on 8 volunteers. Etozolin and Vs show a dose dependent diuretic effect: 400 mg etozolin being equipotent to 75 mg Vs and 1200 mg etozolin being 2.8 times more effective than 75 mg Vs. Both substances show maximum effect 2 to 4 h after application, no differences from placebo being demonstrable 10 h after administration. There were no significant differences in potassium excretion between etozolin, Vs and placebo, etozolin and Vs causing, however, a significant excretion of sodium and chloride in contrast to placebo. Etozolin seems to act like a diuretic of the sulfonamide-type in respect to the profile of daily urinary output whereas resembling the loop-diuretic type in respect to urinary volume elimination.