L-Threo-DOPS, a noradrenaline (NA) precursor, produced a dose-dependent increase in the volume of urine in mice and rats. It also increased the total output of sodium and chloride ions, but not the excretion of potassium ion. Treatment with peripheral decarboxylase inhibitors antagonized not only the diuretic action, but also the increase in the concentration of kidney NA produced by L-threo-DOPS. These results suggest that the diuretic action of L-threo-DOPS might not be due to its direct action, but largely to NA formed by its decarboxylation in the kidney.