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.