1. Twenty-two patients with renovascular hypertension and significant lateralization of renin secretion were investigated with respect to arterial and renal venous plasma noradrenaline and dopamine concentrations before and 30 min after two different blood pressure-lowering procedures (enalaprilat or dihydralazine intravenously).
2. In one group of patients (n = 10) blood pressure reduction by 7.5 mg of dihydralazine caused stimulation of the renin-angiotensin system and increased heart rate (P < 0.01) as well as arterial and renal venous plasma noradrenaline and dopamine concentrations (P < 0.01 for both). The elevation of the plasma dopamine concentration was more pronounced than that of noradrenaline, yielding increased dopamine/noradrenaline ratios both peripherally (P < 0.05) and in the renal veins (P < 0.05).
3. In the second group of patients (n = 12) a comparable reduction in blood pressure elicited by 1.25 mg of enalaprilat was not accompanied by alterations in heart rate or in noradrenaline or dopamine levels in arterial plasma or renal venous plasma from either kidney.
4. Thus, inhibition of angiotensin-converting enzyme does not elicit the expected sympathetic counter-regulatory response to blood pressure reduction; the mechanism(s) involved remains to be clarified.