The activity of renal Na-K-ATPase was compared in hypertension-prone (SBH) and hypertension-resistant (SBN) Sabra rats on regular sodium intake and 2–3 weeks after a high sodium diet. ATPase activity was determined in single nephron segments by a micromethod. The activity profile was found to be similar in the two substrains on both regimens. Following high sodium intake there was a significant increment of Na-K-ATPase activity which was limited to the medullary thick ascending limb in the two substrains. The results clearly indicate a lack of relationship between renal Na-K-ATPase activity and proneness or resistance to hypertension in this experimental model.