Objective:The present study examines the effect of angiotensin converting enzyme inhibition on the renal haemodynamic and sodium excretory responses to noradrenaline in man.Design:We studied the effects of intravenous noradrenaline (0.075 (J.g/kg per min) and enalapril pretreatment (5 mg/day for 5 days), alone and in combination, on urinary sodium excretion, effective renal plasma flow, glomerular filtration rate and segmental tubular function in nine normal subjects.Methods:The subjects were studied during maximal water diuresis. The clearance of inulin and of para-aminohippurate were used to estimate the glomerular filtration rate and effective renal plasma flow, respectively. Segmental tubule handling of sodium was assessed by the lithium clearance method.Results:Noradrenaline alone decreased urinary sodium excretion (P<0.01) and the effective renal plasma flow (P<0.01) without altering the glomerular filtration rate. Enalapril pretreatment significantly attenuated this fall in sodium excretion (P<0.05) and effective renal plasma flow (P<0.05), and had a similar attenuating effect on the noradrenaline-induced decrease in the fractional excretion of lithium. The pressor response to noradrenaline infusion was not, however, influenced by the enalapril pretreatment.Conclusions:Enalapril blunts the renal vasoconstrictive effect and the antinatriuretic effect of noradrenaline in man. Our results indicate that there is an important interaction between the sympathetic nervous system and the renin-angiotensin system in the kidneys in man.