Rats fed a low potassium diet were studied before and after KCl infusion to determine whether potassium recycling in the renal medulla accelerates potassium excretion by increasing delivery of sodium, water, and potassium to the distal tubule. Fluid samples were obtained from the end-proximal and beginning-distal tubule before and at the same sites after KCl loading by re-collection micropuncture and were compared with samples obtained from similarly fed control animals not infused with KCl. Potassium excretion increased in the KCl group from 3 to 48% but remained low in the controls. Fractional sodium and water delivery to end-proximal and beginning-distal tubule increased with time equally in both groups. Potassium reabsorption in Henle's loop fell from 75 to 58% (P less than 0.005) after KCl infusion but not significantly in the controls (from 77 to 75%). Fractional potassium delivery to the distal tubule increased from 12 to 26% (P less than 0.005) in the KCl group, which could account for over half the potassium excreted. The increase in controls from 12 to 17% was significantly less (P less than 0.02). These findings suggest that potassium recycling reduces potassium reabsorption in Henle's loop, enabling the loop to participate with the distal and collecting tubule in accelerating urinary excretion of an acute potassium load.