Hydrochlorothiazide (HCT), 25 or 50 mg a day, alone and in combination with triamterene (T), 50 and 100 mg, respectively, or amiloride (A), 2.5 and 5 mg, respectively, was examined for effects on daily urinary potassium and sodium excretion. Daily sodium excretion was increased 1.5-2-fold with a double dose of the combined drugs as compared with their single dose. T and A produced additive effects on the natriuretic action of HCT. Lower doses of HCT and T failed to prevent urinary potassium loss, but their potassium-sparing effect was shown with their double dose. This effect was largely displayed by A given in a single or double dose. Lower serum potassium levels were seen with all the drugs, but this was less marked with HCT combined with T or A than with HCT monotherapy. Their double dose reduced serum potassium levels to an insignificantly greater extent. In some cases, the elderly patients developed hyperkalemia during the combined therapy. Thus, with the routine dose ratios, A showed a slightly higher potassium-sparing effect than did T. Some patients taking HCT in combination with T showed reddish-brown crystals and casts in the urinary sediment, which may indicate its tubular interstitial damaging action. Consequently, use of the combined drugs is more preferable than HCT monotherapy, and HCT in combination with A is likely to be more preferable than that with T.