The effects of X and T in different doses alone or in combination on renal sodium and potassium excretion were examined in conscious rats. In acute experiments, the combination of both diuretics produced an approximately additive effect on sodium excretion. Addition of T to X in different dose relations (X:T = 1:0.5, 1:1 or 1:1.5) prevented the X-induced potassium loss. These effects were more pronounced at the highest dose of T; the difference to the other combinations, however, was small. In chronic experiments, when the diuretics were administered daily for 13 days, both combinations (X:T = 1:1 and X:T = 1:1.5) resulted in a nearly equilibrated potassium balance at the end of this period. In shorter intervals, however, sometimes considerable oscillations in potassium excretion occured.