Hyperglycemia and diabetes have been shown to increase diacylglycerol (DAG) level and activate protein kinase C (PKC) activity in the vascular tissues, possibly altering vascular function. We have characterized the effects of D-α-tocopherol (vitamin E) on PKC activities and DAG levels in rat aortic smooth muscle cells (ASMCs) cultured with elevated glucose levels as well as in the vascular tissues obtained from control and diabetic rats. In ASMCs, the specific PKC activity from the membraneous fraction and total DAG level were increased by 31 ± 4% (P < 0.05) and 50 ± 7% (P < 0.05), respectively, when the glucose levels were changed from 5.5 to 22 mmol/1. The addition of D-α-tocopherol and another lipophilic antioxidant, probucol, prevented the glucose-stimulated increases in DAG level and PKC activity. By immunoblotting studies, D-α-tocopherol treatment was able to reduce the enhancement of PKC (ill isoform in the membraneous fraction isolated from ASMCs. Comparing streptozotocin-induced diabetic rats with their nondiabetic controls, both membraneous-specific PKC activities and total cellular DAG levels were increased in aorta by 162% (P < 0.05) and 60% (P < 0.05), respectively. Intraperitoneal injection of D-α-tocopherol (40 mg/kg) every other day prevented the increases in membraneous-specific PKC activities and total DAG levels in parallel with a significant increase of D-α-tocopherol contents in the aorta and plasma. These findings have demonstrated that D-α-tocopherol can prevent the activation of PKC activities in the vascular cells and tissues induced by hyperglycemia by lowering DAG levels, possibly via its antioxidant effect.