Succinic acid monomethyl ester (SAM) was recently proposed as an insulinotropic tool in non-insulin-dependent diabetes mellitus. The present study was designed to define whether SAM has the vascular effect in thoracic aorta of streptozotocin (STZ)-diabetic rats. (1) Body weights of diabetic rats were significantly increased after SAM treatment (P < 0.05). (2) Ten-day SAM treatment did not significantly affect blood glucose levels in SAM-treated control and SAM-treated STZ-diabetic rats. (3) Maximum tension responses to noradrenaline and KCI (80 mmol l(-1))were not significantly different among all the experimental groups. (4) pD(2), (-log EC50) values for noradrenaline of untreated diabetic rats were significantly less than those of controls, SAM-treated control and SAM-treated diabetic rats (P < 0.01, P < 0.001 and P < 0.05, respectively). SAM treatment normalized the decreased sensitivity of noradrenaline response in diabetic rats. (5) Fast, slow and total components of responses to noradrenaline (10(-5) mol l(-1) congruent to EC90) were not significantly different among all the experimental groups. (6) There were no significant differences between aorta precontracted with noradrenaline from controls and STZ-diabetic (untreated and SAM-treated) rats in pD(2), values and the potency of maximum relaxation to acetylcholine or in pD(2) values to sodium nitroprusside. In conclusion, 10-day SAM treatment increases the sensitivity of diabetic-aortic rings to noradrenaline compared to untreated diabetic control rats. (C) 1998 The Italian Pharmacological Society.