The long-term effect of level of protein intake on calcium metabolism, renal function and renal acid excretion was determined during a 75-day metabolic study. Six women consumed a diet containing either 46 or 123 g protein for 60 days; they then consumed the alternate diet for 15 days. Calcium, phosphorus and magnesium intakes were maintained constant at 500, 900 and 350 mg, respectively, throughout the 75-day study. Urinary calcium was remarkably constant with time at both levels of protein intake but was approximately twice as high when the 123 g protein diet was consumed. Level of protein intake had no effect on calcium absorption; the increase in urinary calcium found when the high protein diet was given, therefore, caused a markedly negative calcium balance. Glomerular filtration rate (GFR) and renal acid excretion were higher and fractional renal tubular reabsorption of calcium was lower when the high protein diet was given. The hypercalciuria caused by the high protein intake was due primarily to the decrease in fractional tubular reabsorption of calcium and, to a lesser extent, to the increase in GFR. Neither GFR, fractional renal tubular reabsorption of calcium nor any of the components of renal acid excretion exhibited any tendency to change with time over the 60-day experimental period.