1. Dose—response curves for the pressor activity of angiotensin II have been determined in unanaesthetized rats receiving diets containing 2·5% (w/w) or 0·007% (w/w) sodium and administered in various sequences.
2. Dose—response curves were shifted to the left in rats on a high-, compared with a low-, sodium intake. This response was maintained for 7 days on changing from high to low sodium.
3. There was no difference in the relation between the fall of cardiac output and the rise of blood pressure in any of the experimental groups.
4. Dose—response curves for peripheral resistance showed the same directional change as seen for the pressor response in rats on high- and low-sodium diets. Since depression of cardiac output was proportional to the pressure rise, the absolute change in peripheral resistance was greater than the blood pressure response. The proportional changes were similar.
5. It is concluded that alterations in the pressor response to angiotensin caused by changes in sodium loading are attributable to changes in peripheral resistance and not to changes in the cardiac output response to the acute rise in blood pressure.