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.

1. In 12 week-old stroke-prone spontaneously hypertensive (SPSH) rats and in normotensive Wistar-Kyoto (WKY) rats plasma concentration of [arginine]vasopressin (AVP) and the AVP content in various brain areas were measured by radioimmunoassay. 2. In hydrated SPSH rats plasma AVP was lower than in WKY rats. In the hypothalamus and in the brain stem, but not in the pituitary, the content of AVP was reduced. 3. After a dehydration period of 48 h plasma AVP rose similarly in both SPSH and WKY rats. However, in the pituitary the AVP content was significantly lower in SPSH than in WKY rats. In the hypothalamus and in the brain stem, AVP content was not significantly influenced by dehydration. 4. It is concluded that in the hydrated stage the secretion of AVP and its synthesis in the hypothalamus are reduced in SPSH rats. However, the SPSH rats still respond satisfactorily to the strong stimulus of severe dehydration.