1. Changes in arterial blood pressure, blood angiotensin I, plasma angiotensin II and plasma angiotensin III were measured in conscious sodium—depleted dogs after infusion of captopril, an orally active inhibitor of converting enzyme.

2. Angiotensins II and III were measured after chromatography to remove angiotensin I, which increased in concentration after inhibition of converting enzyme and which interfered in the direct assay for angiotensin II.

3. Infusion of captopril at 20, 200, 2000 and 6000 μg h−1 kg−1, each for 3 h, produced a rapid fall in blood pressure and in concentration of angiotensin II. Angiotensin II was undetectable at 6000 μg h−1 kg−1 (mean pre-infusion value for all samples was 39 ± sd 15 pmol/I, n = 14)

4. The percentage fall in blood pressure correlated with the percentage fall in plasma angiotensin II (r = 0.65, P<0.001)

5. These results suggest that the initial fall in blood pressure may be mediated in part by the suppression of angiotensin II.

6. Blood angiotensin I concentration rose with each rate of infusion of drug to a maximum 16-fold increase at 6000 μg h−1 kg−1 (26−416 pmol/l). The rise in angiotensin I was inversely related to the fall in angiotensin II (r = −0.68, P<0.001)

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