The effect of potassium on sodium chloride uptake into rabbit renal medullary thick ascending limb of Henle's loop (mTALH) cells was studied to assess whether K participates in the Na-Cl cotransport system. Na uptake into the mTALH cells was inhibited 70% at 3 min by 1 mM furosemide. The total and furosemide-sensitive Na uptake was stimulated by Cl. Additionally, Cl uptake into the mTALH cells was stimulated by Na gradients and inhibited 42% at 3 min by 1 mM furosemide. Na uptake was studied in the presence of 0,5, or 140 mM external K gradients. Na uptake was similar in the absence and presence of K. Additionally, furosemide inhibited Na uptake as effectively in the absence or presence of K. Similar studies were conducted to study the effects of Na on 86Rb uptake. Na did not stimulate 86Rb uptake. The uptake of 86Rb was similar in the presence of 0,5, or 140 mM Na gradients. Furosemide had no significant inhibitory effect on 86Rb uptake. Barium (5 mM), an inhibitor of K conductance pathways, inhibited total 86Rb uptake by 19%. In the presence of 5 mM BaCl2, Na still did not have a stimulatory effect on 86Rb uptake. The results confirm the existence of a Na-Cl cotransport system in mTALH cells, but a direct effect of K on the NaCl cotransport system could not be demonstrated under the experimental conditions we used.