The original lead-trapping method for demonstrating Na⁺−K⁺-ATPase activity was discredited because of the effect that lead ions can have on the substrate and on the enzyme. Current methods, that measure this activity by the related K⁺-dependent phosphatase activity, do not appear to measure activity that is known, from microchemistry, to occur in proximal convoluted tubules. The disadvantages of using lead appear to have been overcome by the use of a new reagent in which the lead is complexed with ammonium citrate ions; phosphate, liberated enzymatically, successfully competes with these ions. The activities of total ATPase and of the ouabain sensitive Na⁺−K⁺-ATPase have been measured in three regions of the nephron in the guinea-pig and in the rat. The relative activities found, by this method, in the different regions of the latter, appear to be comparable with results found by others, using microchemical methods applied to isolated regions of the nephron.