There is conflicting evidence regarding the effects of high protein intake on kidney health, especially as it relates to age. We investigated the short-term effects of a high-protein diet on kidney function and systemic acid-base homeostasis in older compared to younger adults. The subjects were healthy men and women either between the ages of 25 and 40 years (n=12) or 55 and 70 years (n=10). They underwent a two-period crossover trial with each period consisting of 2 weeks of usual diet followed by a 1-week experimental diet. During the experimental diet period subjects consumed metabolic meals that provided either low protein content (0.5 g protein/kg/day) or high protein content (2.0 g protein/kg/day). Outcome measures included blood and urine markers of renal function and acid-base balance. An analysis of variance was used to assess differences between age groups with respect to experimental diet. The older group, mainly women, showed an increase in glomerular filtration rate after the high-protein compared to low-protein diet; the younger group did not. Urinary pH was significantly lower, and ammonium excretion was significantly higher after the high-protein diet in both age groups, but neither group developed a clinically detectable acidosis after the week of receiving a high-protein diet.