ADMA (asymmetric dimethylarginine), an endogenous inhibitor of nitric oxide synthase, is considered a major risk factor for cardiovascular disease and progression of renal disease. In the present study we aim to investigate the effect of acute variations in plasma glucose and insulin on plasma ADMA levels in young people with T1D (Type 1 diabetes). Fifteen young patients (ten males) with T1D, median age 18.3 (13.2-24.4) years, HbA(1c) (glycated haemoglobin) 9% (6.4-13.6%), underwent an overnight (18:00-08:00 hours) variable insulin infusion for euglycaemia, followed by a hyperinsulinaemic-euglycaemic clamp (08:00-12:00 hours). Blood samples were collected every 15 min for determination of ADMA, SDMA (symmetric dimethylarginine), valine, phenylalanine, arginine, creatinine and glucose. Insulin levels were assessed every 30 min. During the overnight period, glucose levels increased following the evening meal. In response to the protein intake there was a significant increase in ADMA, arginine, valine, phenylalanine and creatinine. For the remaining part of the night, glucose levels progressively decreased reaching 5 mmol/l by 04:00 hours. ADMA and SDMA did not change significantly. During the hyperinsulinaemic clamp, a significant fall in ADMA was observed, from 0.468+/-0.056 to 0.364+/-0.050 micromol/l (P<0.001). A significant fall was also found in SDMA, valine, phenylalanine, arginine and the ADMA/SDMA ratio (all P<0.001), but not in creatinine levels. No correlation was found between insulin sensitivity and ADMA. We conclude that acute changes in glycaemia do not significantly affect plasma ADMA levels whereas infusion of insulin significantly reduces ADMA, suggesting an important role for insulin in the regulation of this cardiovascular risk factor.