1. Total and Na + ,K + -ATPase activities have been measured in sections (10 μm thick) from blocks of rat kidney cultured for 5 h at 37°C, pH 7.5, in Glasgow Eagle's Minimum Essential Medium. 2. Synthetic [arginine]vasopressin ([Arg]VP) stimulated ATPase activity in the thick tubular cells of the renal outer medulla over the concentration range 0.01-10 fmol/l, but failed to affect ATPase activity in the proximal and distal convoluted tubules of the cortex. 3. The increase in medullary total ATPase activity induced by [Arg]VP and its analogues was wholly due to stimulation of Na + ,K + -ATPase activity. 4. Stimulation of medullary ATPase activity was blocked by [Arg]VP antiserum. 5. The [Arg]VP analogues desmopressin, [lysine]vasopressin, [arginine]vasotocin and oxytocin stimulated medullary Na + ,K + -ATPase activity, the three last-named analogues being considerably less potent than [Arg]VP.