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Stimulation of proximal tubular fluid reabsorption by peritubular angiotensin II (Ang ll) was examined by split-drop micropuncture in 5- and 12-week-old spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto rats (WKY). In WKY, the maximum stimulation occurred at 10-11 mol/l and the response did not vary with age. In 5-week-old SHR, the dose-response relationship was similar in shape and in the extent of the maximum response but was shifted one half-logarithmic step to the right, indicating decreased sensitivity to Ang ll. In contrast, the dose-response relationship was shifted one half-logarithmic step to the left in 12-week-old SHR compared with WKY. Alterations in the responsiveness of the proximal tubule to Ang ll in young SHR could contribute to sodium retention observed during development of hypertension in these rats.