1. The purpose of the present study was to determine the relationship between endothelial dysfunction and the endogenous inhibitor of nitric oxide synthase NG,NG'-asymmetric dimethylarginine (ADMA) in aged rats. 2. Studies were performed in male adult Sprague-Dawley rats (6 months old; n = 8) and in aged rats (20 months old; n = 8). Serum levels of ADMA and L-arginine were measured by high-performance liquid chromatography and responses of endothelium-intact aortic rings to acetylcholine (ACh) were tested. Nitric oxide synthase activity in kidney tissue and serum concentrations of nitrite, a stable end-product of nitric oxide, were assayed and serum contents of malondialdehyde, derived from lipid peroxidation and serum lipid and creatinine level were determined. 3. Serum levels of ADMA increased significantly in aged rats compared with adult rats (P < 0.01), whereas serum levels of L-arginine were similar in both groups (P = NS). Accordingly, the ratio of L-arginine/ADMA in old rats was lower than that in young rats (P < 0.01). Endothelium-dependent relaxation responses to ACh in aortic rings from aged rats were impaired and these impaired responses were improved by pre-incubation of aortic rings with L-arginine. 4. Nitric oxide synthase activity in the kidney, together with serum concentration of nitrite, was significantly decreased and serum contents of malondialdehyde, cholesterol and triglycerides were increased in old compared with young rats. However, the serum creatinine level was not significantly different between adult and aged rats. 5. Endogenous ADMA may be a contributor to age-related endothelial dysfunction and increases in endogenous ADMA may be linked to lipid peroxidation in aged rats.