The possible presence of lithium transport beyond the proximal tubule was examined by measuring lithium excretion after administration of triamterene, a potassium-sparing diuretic, exclusively acting in the cortical collecting tubule. Eight young and healthy volunteers were studied on two occasions during maximal water diuresis. After obtaining baseline values triamterene (100 mg orally) or placebo was administered, and measurements continued for 4 hours. Creatinine clearance was used as a marker of glomerular filtration rate, and phosphate excretion was used as an additional marker of proximal sodium transport. Compared to placebo (P), triamterene (T) caused a significant increase in fractional excretion of sodium (P, 0.74 +/- 0.08%; T, 1.73 +/- 0.24%, mean +/- SEM; P less than 0.01), and lithium (P, 21.2 +/- 1.3%; T, 27.5 +/- 1.5%; P less than 0.01), whereas fractional excretion of phosphate remained unchanged (P, 9.8 +/- 1.3%; T, 9.4 +/- 1.5%; P = NS). These results indicate that lithium is transported in the cortical collecting tubule, and provide further evidence that the use of lithium as a marker of purely proximal tubular sodium transport is of limited value.