1. Noradrenaline sensitivity and relaxation to acetylcholine were investigated in the isolated perfused mesentery and in aortic rings of control and streptozotocin (STZ)-induced (50 mg kg-1) diabetic Charles River rats. 2. In addition, noradrenaline sensitivity and acetylcholine relaxation were similarly assessed in streptozotocin-induced diabetic rats treated from the time of onset of diabetes with the aldose reductase inhibitor, ponalrestat (100 mg kg-1 day-1). 3. The untreated diabetic rats (2-10 weeks after injection of STZ) demonstrated enhanced vascular sensitivity to noradrenaline in the perfused mesenteric arterial tree, compared with age matched controls (pEC50 [-log concentration (M)]: diabetic 5.62 +/- 0.09, n = 18, versus control 5.23 +/- 0.07, n = 16, P < 0.01). 4. Acetylcholine-induced relaxation was significantly impaired in the perfused mesentery of the diabetic animals compared to controls (pED50 [-log dose (mol)]: diabetic 9.87 +/- 0.10, n = 20, versus controls, 10.29 +/- 0.09, n = 20, P < 0.05). 5. In contrast, the aortic ring preparations demonstrated no significant functional differences between the diabetic and control groups in response to either noradrenaline (pEC50: diabetic 7.66 +/- 0.08, n = 15, versus controls 7.55 +/- 0.06, n = 15, NS), or acetylcholine (pEC50: diabetics 7.30 +/- 0.06, n = 15, versus controls 7.40 +/- 0.09, n = 15, NS). 6. Treatment with the aldose reductase inhibitor, ponalrestat, did not affect the increased vascular reactivity to noradrenaline, or impaired relaxation to acetylcholine in the perfused mesentery.