Objective To examine the effects of angiotensin converting enzyme (ACE) inhibition on renal and systemic haemodynamics as well as on humoral regulators, under resting conditions and during mental stress in 20 normotensive and 20 mildly hypertensive subjects. Methods All of the subjects received either 25 mg cilazapril or placebo once a day, in a randomized, double-blind, crossover trial for 1 week, followed by a 2-week washout period before the alternative regimen was given. We measured renal blood flow with para-aminohippuran, glomerular filtration rate with inulin, cardiac output by impedance cardiography and blood pressure and heart rate by an oscillometric method. We also monitored effects on plasma renin activity, aldosterone, catecholamines and atrial natriuretic peptide. Mental stress consisted of a long-lasting, time-reaction device, thereby provoking activation of the sympathetic nervous system. Results At rest ACE inhibition lowered mean arterial pressure (92 ± 10 versus 98 ± 9 mmHg), increased renal blood flow (803 ± 109 versus 707 ± 93 ml/min) and the renal fraction of cardiac output (25.9 ± 2.5 versus 23.5 ± 2.5%) and decreased the filtration fraction (17.9 ± 2.5 versus 19.8 ± 2.7%) in hypertensive but not in normotensive subjects. Sympathetic activation by mental stress leading to a transient increase in blood pressure did not alter significantly the effects of ACE inhibition on renal and systemic haemodynamics, in normotensive or in hypertensive subjects, although a tendency towards attenuation of the rise in glomerular filtration rate was noted in hypertensives (7.2 ± 1.0 versus 5.1 ± 0.8%). ACE inhibition led to increased plasma noradrenaline at rest but not during mental stress in hypertensive patients. Conclusion ACE inhibition in patients with mild hypertension increased selectively renal perfusion, which is conserved during mental stress without persistent effects on the sympathetic nervous system. Thus, mental stress as a correlate of daily life stress appeared not to confound the selective renal vasodilatory effect of ACE inhibitors.