Samples of proximal and distal tubular fluid were collected from rats maintained on a control, a low-K, or a high-K, low-Na diet. All animals received inulin-C14. Plasma (P) and tubular fluid (TF) were analyzed for Na and K by dual-channel microflame photometry and assayed for radioactivity. Transtubular electrical potential differences were measured by means of glass microelectrodes. Mean TF/P ratios for potassium in the proximal tubule were slightly below unity in all groups of animals. A comparison of the relative increase in K and inulin-C14 along the distal tubule indicates: 1) net movement of potassium into the tubular lumen in most control animals; 2) net movement of K into the tubular lumen of high-K, low-Na, sulfate-loaded animals, and in dichlorphenamide-treated animals on a control diet; and 3) the possibility of continued net reabsorption of potassium along the distal tubule and, particularly, the collecting duct in animals kept on a low-K diet. Distal tubular entry of potassium occurs down an electrochemical potential gradient.