Potassium bicarbonate, but not sodium bicarbonate, reduces urinary calcium excretion and improves calcium balance in healthy men. Previous studies demonstrated that the administration of NaHCO₃ or sodium citrate had either only a small effect to reduce urinary Ca excretion or no effect, but that potassium citrate significantly reduced urinary Ca excretion. In order to further evaluate and compare the effects of NaHCO₃ and of KHCO₃, we performed ten metabolic balances in healthy men during 18 control days, 12 days of NaHCO₃, 60 mmol/day and 12 days of KHCO₃, 60 mmol/day. Six subjects were fed a low Ca diet (5.2 ± 0.7 SD mmol/day) and three of these were also given calcitriol (0.5 µg 6-hourly). Four subjects ate a normal Ca diet (19.5 ± 1.3 mmol/day). For all 10 subjects, KHCO₃ administration reduced urinary Ca excretion from control by -0.9 ± 0.7 mmol/day, P < 0.001. Net intestinal Ca absorption did not change detectably so that Ca balances became less negative by a +0.9 ± 0.9 mmol/day; P = 0.01. KHCO₃ administration was also accompanied by more positive PO₄ and Mg balances. NaHCO₃ administration had no significant effect on urinary Ca excretion or Ca balance. NaHCO₃ and KHCO₃ administration were accompanied by equivalently more positive Na or K balances, respectively and equivalently more negative acid balances (HCO₃ retention). Neither NaHCO₃ or KHCO₃ altered fasting serum HCO₃ concentrations, blood pH, serum 1,25-(OH)₂-D or PTH concentrations. We conclude that KHCO₃ promotes more positive Ca balances by either enhancing renal Ca retention or skeletal Ca retention or both.