1 The renal haemodynamic responses to a close arterial infusion of noradrenaline (29.7-177.9 nmol kg-1 h-1) were measured in rats anaesthetized with pentobarbitone. Systemic blood pressure was unaffected by noradrenaline infusion at this dose level. Renal blood flow was significantly reduced by 16% while glomerular filtration rate remained unchanged. These responses resulted in a rise in filtration fraction of some 10%. 2 In a separate group of animals, noradrenaline infusion in this manner and at similar dose rate increased plasma renin activity approximately 3 fold. 3 Continuous infusion of the angiotensin converting enzyme inhibitor, teprotide (3.36 mumol kg-1 h-1), had no measurable effect on systemic blood pressure, renal blood flow, glomerular filtration rate or filtration fraction. 4 Infusion of noradrenaline into these animals receiving teprotide caused a significant fall in renal blood flow of 16%. There was a fall in glomerular filtration rate of some 17% which was significantly different from the response observed in the animals not receiving teprotide. There was a consequent small but insignificant fall in filtration fraction. 5 These data show that the regulation of glomerular filtration rate in response to the vasoconstrictor drug, noradrenaline, is partly mediated via the renin-angiotensin system. They provide evidence for a role of intrarenal angiotensin II in regulating glomerular filtration by causing efferent arteriolar vasoconstriction.