It has been observed that phenylephrine induced mean arterial pressure (MAP) changes in rats cause different values of aortic stiffness, measured by pulse wave velocity (PWV), at the same MAP during increasing, compared to decreasing pressure. The reason for this hysteresis in the MAP-PWV relationship is not understood. This study evaluates the influence of a variety of cardiovascular factors on PWV. Five, 12 week old Wistar-Kyoto rats were anaesthetised and two pressure sensors introduced into the thoracic and abdominal aorta. Pressure was increased with intravascular phenylephrine infusion (30μg/kg/min), and allowed to return to baseline following cessation of infusion. This was repeated following intravascular hexamethonium bolus (20mg/kg), blocking autonomic activity to quantify the role of the sympathetic system. PWV, thoracic MAP, pulse pressure, form factor, maximum slope of pressure (dP/dt, surrogate measure of cardiac contractility), and R to R interval from the electrocardiogram were calculated for each individual pulse. These variables along with hexamethonium presence, and dynamic MAP direction were entered into a stepwise linear regression model (dependent variable: PWV). MAP, R to R interval, sympathetic activity, pulse pressure, dynamic MAP direction and dP/dt were significant predictors (Table). The results indicate that cardiovascular parameters other than MAP, especially pulse pressure, and notably sympathetic activity and R to R interval, are predictive of PWV. MAP direction remained in the model, indicating that other factors are at play, or that the aorta displays a short term viscoelastic memory. Further studies are required to isolate each of these factors and their effect on PWV.