The effect of K+ homeostasis on distal nephron acidification was investigated by comparing isolated perfused kidneys from rats fed a K+-free or high-K+ diet for 1 week. H+ secretary capacity of the distal nephron was determined by perfusion of kidneys at pH 6.8 and providing saturating quantities of urinary buffer in the form of creatinine. Under these conditions, kidneys from rats with depleted K+ had a lower urine pH level and a higher excretion rate of titratable acid. Acid secretion below a pH of 6.0, an index of H+ secretion by the distal nephron, was also significantly greater in the rats with depleted K+. After addition of 10(-5) mol/L amiloride hydrochloride to the perfusate, urine pH levels remained lower and acid secretion below a pH of 6.0 remained higher in the group with low compared with high dietary K+. Thus, K+ depletion increases the intrinsic distal nephron H+ secretory capacity, and the mechanism appears to be independent of distal nephron Na+ transport. Additional comparisons with kidneys from control rats and rats with long-term metabolic acidosis and alkalosis suggest that the predominant effect on acidification is stimulation by K+ depletion rather than suppression by a high K+ intake. Because our data suggest that K+ depletion per se stimulates the distal nephron H+ transport system, it seems likely that the high urine pH level that accompanies K+ depletion in vivo is attributable entirely to the K+ depletion-induced increase in the rate of renal NH3 production.