SUMMARY To determine the relative importance of hormonal factors in mediating the hypotensive response to converting enzyme inhibition (CEI), plasma renin activity (PRA), angiotensin II, and bradykinin responses to SQ20,881 were measured in 20 supine patients with essential hypertension in balance on a 10 mEq sodium diet. Patients were divided into two groups according to their diastolic blood pressure response: responders had a decrement in diastolic pressure which exceeded 9 mm Hg, toe upper value of the 95% confidence limits for normotensive patients studied under similar conditions; nonresponders did not. Compared to the nonresponders, responders not only had a higher control PRA (8.7 ± 1.7 ng/ml/hr vs4.8 ± 2.1, p < 0.05)and larger decrement in plasma angiotensin II (18.7 ± 4.9 pg/ml vs 3.2 ± 1.7, p < 0.01), but also had a higher control bradykinin (3 2 ± 0.7 ng/ml vs 1.1 ± 0.2, p < 0.05) and larger increment in bradykinin (4.5 ± 13 ng/ml vs 1.0 ± 0.4, p < 0.05) following SQ20.881. Because SQ20.881 altered both angiotensin II and bradykinin concentrations, we assessed the contribution of blockade of angiotensin II formation by administering angiotensin II infusions to seven responders during converting enzyme blockade, with the angiotensin II dose adjusted to restore diastolic pressure to control levels. The plasma angiotensin II level required to return blood pressure to control was 45 ± 15 pg/ml higher than the control plasma angiotensin II level (p < 0.01), suggesting that some other factors), perhaps bradykinin, are also responsible for the hypotensive response to converting enzyme inhibition.