The effect of acute arterial hypertension on blood-brain barrier (BBB) permeability was studied in streptozocin-induced diabetic rats using Evans blue as a barrier tracer Pour groups of rats were studied: Group 1, normotensive normoglycemia; Group II, normotensive + diabetes mellitus; Group III, arterial hypertension + diabetes mellitus; Group IV arterial-hypertension + normoglycemia. During adrenaline-induced acute arterial hypertension the mean arterial blood pressure increased in both non-diabetic and diabetic animals. Changes in BBB permeability were observed in 52% of the non-diabetic rats, and in 72% of the diabetic rats after adrenaline-induced acute arterial hypertension. Mean levels of Evans blue in the whole brain were found to be 0.63 + 0.1 mg% in nondiabetic and 0.90 +/- 0.2 mg% in diabetic rats. The difference between the non-diabetic and the diabetic rats was found to be statistically significant (P < 0.01). From these results it was suggested that the extravasation of Evans blue albumin is more pronounced in the brains of diabetic rats in comparison with non-diabetic rats after adrenaline-induced acute hypertension, which is indicative of changes in BBB permeability due to diabetes mellitus.