Potassium secretory flux (J(K)) by the distal nephron is regulated by systemic and luminal factors. In the present investigation, J(K) was measured with a double-barreled K+ electrode during paired microperfusion of superficial segments of the rat distal nephron. We used control solutions (100 mM NaCl, pH 7.0) and experimental solutions in which Cl- had been replaced with a less permeant anion and/or pH had been increased to 8.0. J(K) increased when Cl- was replaced by either acetate (similar to37%), sulfate (similar to32%), or bicarbonate (similar to62%), and also when the pH of the control perfusate was increased (similar to26%). The majority (80%) of acetate-stimulated J(K) was Ba2+ sensitive, but furosemide (1 mM) further reduced secretion (similar to10% of total), suggesting that K+-Cl- cotransport was operative. Progressive reduction in luminal Cl- concentration from 100 to 20 to 2 mM caused increments in J(K) that were abolished by inhibitors of K+-Cl- cortransport, i.e., furosemide and [( dihydroindenyl) oxy] alkanoic acid. Increasing the pH of the luminal perfusion fluid also increased J(K) even in the presence of Ba2+, suggesting that this effect cannot be accounted for only by K+ channel modulation of K+ secretion in the distal nephron of the rat. Collectively, these data suggest a role for K+-Cl- cotransport in distal nephron K+ secretion.