The content and the properties of (Na+, K+)-ATPase in the thick ascending limb of the loop of Henle (TAL) are related to the rate and the characteristics of the transport of NaCl which has been measured in the isolated perfused cortical portion of the TAL by Burg and Green (Am. J. Physiol, 224, 659, 1973) and in the medullary portion of the TAL by Rocha and Kokko (J. Clin. Invest. 52, 612, 1973). It is concluded that the ouabain-sensitive, active transport of NaCl across the epithelium consists of primary active transport of Na and secondary active transport of Cl and that it is driven by the sodium pump or (Na+, K+)-ATPase.