1. Blood pressure, renin concentration and angiotensin II were measured in unanaesthetized two-kidney one-clip hypertensive rats at 1 and 2 days, at weekly intervals up to 10 weeks and at 15 and 20 weeks after clipping.

2. Compared with values in sham-operated rats, renin and angiotensin II were initially increased at 1–2 days but were then suppressed between 2 and 4 weeks to levels similar to that found in sham-operated rats. Between 5 and 20 weeks renin and angiotensin II increased again to high levels.

3. There was a significant correlation between angiotensin II and blood pressure in acute rats 1–2 days after clipping (P < 0.05) and in chronic rats 8–20 weeks after clipping (P < 0.001). There was no difference in the slope of the regression lines but the regression line for the chronic rats was shifted upwards in a parallel manner.

4. The acute hypotensive response (−20.3 ± sd 24.9 mmHg) in 26 chronic rats given converting enzyme inhibitor was related to the basal renin and angiotensin II levels and followed the slope of the angiotensin II/blood pressure regression line for all chronic rats. Only one out of 26 rats reduced its blood pressure to normal levels.

5. In 12 rats at 4 weeks after clipping, when blood pressure was elevated but angiotensin II was suppressed, there was only a small fall in blood pressure (−7.1 ± sd 7.2 mmHg). This also followed the angiotensin II/blood pressure regression line for chronic rats but at the lower end. Blood pressure again was not reduced to normal.

6. These results suggest that renin and angiotensin II are increased up to 20 weeks after clipping, that there is no change in the net vascular responsiveness to endogenous angiotensin II at any stage in this experimental model and that the acute effect of angiotensin II is determined solely by its position in the same dose—response curve. Also with the exception of 1–2 days immediately after clipping the acute effect of angiotensin II plays only a minor, though variable, role in the hypertension and that some other mechanism, as yet undetermined, is of greater importance and begins to have an effect as early as 2 weeks after clipping.

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