The goal of this study was to determine whether the susceptibility of the blood-brain barrier to disruption during acute hypertension is altered in aged rats. Intravital fluorescent microscopy and fluorescein-labeled albumin were used to evaluate disruption of the blood-brain barrier during acute hypertension in adult Wistar rats (6-8 mo) and aged Wistar rats (24-26 mo). Permeability of the blood-brain barrier albumin and by counting the number of microvascular leaky sites under control conditions and during acute arterial hypertension in adult and aged rats. Pressure in pial venules, which are the primary site of disruption of the blood-brain barrier during acute hypertension, and in pial arterioles was measured using a servo-null device. In adult rats, when systemic arterial pressure was increased from 124 +/- 5 (means +/- SE) to 190 +/- 5 mmHg, clearance of albumin increased from 0.10 +/- 0.05 to 1.64 +/- 0.76 X 10(-6) ml/s. In aged rats, when systemic arterial pressure was increased from 116 +/- 4 to 185 +/- 3 mmHg, clearance of albumin increased from 0.10 +/- 0.03 to 1.56 +/- 0.49 X 10(-6) ml/s. Increases in pial venular pressure and pial arteriolar pressure also were similar in adult and aged rats. Thus the findings suggest that the susceptibility of the blood-brain barrier to disruption during acute hypertension is similar in adult and aged rats.