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 microgram 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 less than 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.