Collecting duct transport of fluid, sodium and potassium was studied in rats infused with Ringer solution (5 ml . 100 g body wt-1 . h-1). A terminal segment of surface collecting duct in the exposed papilla was catheterized as far upstream as possible under visual observation. After fluid sampling the same duct was punctured at the same site with a glass micropipette and a second sample was taken. Samples were then obtained from the opening of the duct at the papilla tip by both catheter and micropipette. No significant difference between the two collection sites was found in the fraction of filtered sodium, potassium, or fluid remaining in the tubules, independent of the sampling technique used, indicating that volume expansion inhibited salt and water reabsorption. Although fractional fluid and sodium remainders were slighlty higher and potassium remainder lower in upstream micropuncture samples compared to catheterization samples, the good correlation between collections obtained with both techniques suggests that both are equally valid as indicators of transport in terminal collecting ducts.