Introduction Angiotensin-converting enzyme (ACE) inhibitors have been shown to improve endothelial function in Type 1 diabetes. However, the potential of ACE inhibitors (ACE-I) to enhance the haemodynamic effects of L-arginine (L-arg), the precursor of nitric oxide (NO), has not been evaluated. Furthermore, angiotensin receptor blockers (ARBs), another group of inhibitors of the renin-angiotensin system (RAS), have not been studied in this context. Methods Using a randomised, crossover design, the acute effects of L-arg (200 mg/kg) on blood pressure (BP) and renal haemodynamics were determined in uncomplicated Type 1 diabetic patients before and after three weeks of treatment with the ACE-I ramipril (5 mg/day) or the ARB losartan (50 mg/day). Results L-arg alone did not influence BP or renal haemodynamics. BP responses to L-arg were not modulated by ACE-I or ARB. In contrast to the systemic responses, L-arg induced significant renal vasodilation after treatment with ramipril (p<0.05). Unlike ramipril, losartan did not modulate renal haemodynamic responses to L-arg. L-arg administration was paralleled by increments in plasma L-citrulline levels, determined as a measure of L-arg-induced systemic NO production. These responses were not influenced by RAS inhibitors. No changes in other indicators of the systemic and renal NO production, such as plasma and urinary nitrates/nitrites, were detected in response to L-arg alone or after treatment with RAS inhibitors. Conclusions ACE-Is have greater potential than ARBs to enhance L-arg effects in the kidney in uncomplicated Type 1 diabetes. Neither RAS inhibitor influenced the systemic effects of L-arg. The lack of changes in renal NO indicators parallelling the haemodynamic responses, suggests that the effects of ACE-I on L-arg-induced renal haemodynamic changes could be also attributable to NO-independent mechanisms.