Renal acidification in healthy elderly subjects was studied using the short ammonium chloride loading test. No differences in basal urinary acid excretion or urinary pH between elderly and younger subjects were found. After the acid load, the net acid excretion and the percent excretion of the ingested load in 6 h were much less in the elderly. Also, they did not achieve as low a urinary pH as younger persons. Ammonium excretion was reduced in the elderly volunteers, even after correction for GFR. We observed an inverse correlation between urinary pH and ammonium excretion which is statistically significant in younger, but not in elderly subjects. Our results, therefore, show that with age, the capacity to excrete an acid load is diminished due to a reduction in ammonium excretion, independent of decreased GFR. We also noted a small pH gradient defect in the older persons.