To examine the effect of reducing medullary interstitial solute concentration on sodium and potassium flow at the end of the juxtamedullary descending limb in Psammomys obesus, micropuncture was performed on the exposed left renal papilla. After a control period, furosemide was administered to reduce medullary interstitial solute concentration without altering the delivery of sodium, potassium and water from the proximal tubule. The fraction of filtered sodium remaining at the end-descending limb before (52 ± 4.9%) and after furosemide (55 + 4.2%) was not significantly different, despite a fall in tubule-to-plasma osmolality from 4.32 to 2.00 (p < 0.001). In contrast, the fraction of filtered potassium delivered to the end-descending limb fell from 91 ± 9.0% to 61 + 8.0% (p < 0.001). As expected, the fraction of filtered sodium and potassium remaining at the end of the accessible proximal tubule of the superficial nephron was not changed after the administration of furosemide. While these findings do not provide additional support for the thesis of transepithelial sodium addition to the juxtamedullary descending limb, they strongly suggest that transepithelial entry (secretion) of potassium normally occurs upstream to the juxtamedullary hairpin turn of Psammomys obesus.