Cerebral autoregulation allows cerebral blood flow (CBF) to be maintained in the face of changes in systemic arterial blood pressure (ABP): cerebral vasoconstriction occurs when ABP rises and vice versa. Ageing is associated with impaired autoregulation, the upper limit being reduced, but the mechanisms are not known. The aim of this study was to develop a protocol to reliably assess the upper limit in rats by using phenylephrine (PE) to increase ABP: PE has limited ability to cross the blood brain barrier and α-adrenoreceptor density on cerebral vessels is low. In 14 male Wistar rats (333 ± 6g) anaesthetised with Alfaxan (17-20 mg.kg-1.h-1 i.v.), PE (0.1-60µg.kg-1.min-1) was infused i.v. to cause graded increases in ABP. CBF was recorded via a Transonic probe on the right common carotid artery after ligating the external carotid to minimise extra-cranial flow (RCBF). In Group 1, femoral (muscle) BF was also recorded (n=9); in Group 2, left CBF was recorded without ligation (LCBF; n=5). Vascular resistances (CVR, FVR) were computed as ABP/CBF or FBF. Arterial blood gases and pH were monitored at intervals. All procedures were carried out in accordance with UK legislation. While ABP increased from 125±3 to 141±4 mmHg, RCBF was not constant, but fell from 1.3±0.1 to 1.1±0.1ml.min-1 due to vasoconstriction: RCVR increased from 106±8 to 134±11 RU (P<0.05, unpaired t-test). Thereafter, RCBF achieved a plateau while ABP increased to ~155mmHg and RCVR increased, demonstrating autoregulation. The upper limit of autoregulation, defined as the intersect of two linear regression lines fitted to the ‘plateau’ and ‘rising’ portions of mean RCBF was 165mmHg, after which RCBF rose to a maximum of 2.1±0.3ml.min-1. The changes induced in LCBF were similar to those in RCBF, but LCBF was 0.98±0.20ml.min-1 higher (P<0.05, paired t-test) than RCBF, indicating constant extra-cranial flow. FBF tended to rise (1.7±0.3 to 2.5±0.3ml.min-1) while ABP increased, but fell to 1.8±0.1ml.min-1 at the top dose of PE: the trend for FVR to increase did not reach significance (89±15 to 105.6±7.5 RU, P>0.05, one-way ANOVA). PaCO2 and pH remained constant, but PaO2 fell with increasing doses of PE (to 63±3mmHg), suggesting progressive pulmonary vasoconstriction. These data indicate that i.v. PE can be used to assess the upper autoregulatory limit in the rat. However, PE induced an initial cerebral vasoconstriction and fall in CBF before the autoregulatory plateau was apparent. This was not an artefact of ligating the external carotid artery. Further, such initial vasoconstriction did not occur in muscle, suggesting it was peculiar to cerebral circulation: it may be that induction of anaesthesia induced cerebral vasodilatation. We are currently investigating the mechanisms contributing to the upper autoregulatory limit that may change with ageing.