We investigated the reliability of calculating the mean blood pressure (MBP) in spontaneously hypertensive rats (SHR) from the systolic and diastolic blood pressures (SBP; DBP), using a form factor, calculated as: (MBP – DBP)/(SBP – DBP). The mean values of this form factor, as determined by blood pressure curve integration, were 0.459 and 0.450 in awake and anaesthetized SHR, respectively. There was no change in the form factor with pulse frequency. When direct femoral artery MBP measurements (x) were compared with MBP values (y) calculated from tail-cuff measurements of SBP and DBP using the form factor, the linear relationship between the two parameters was: y = 0.91x + 1.5 mmHg (r = 0.98). Actually, direct measurements confirmed that the tail artery MBP was 6–7% lower than the femoral artery MBP. In awake Wistar–Kyoto (WKY) rats, the form factor was 0.468. We therefore concluded that an approximate form factor value of 0.46 could be used in rats to estimate the MBP from known values of SBP and DBP. We further suggest that, because pulse pressure in any given rat remains relatively constant with time, SBP and DBP can be estimated in experiments by initially measuring the pulse pressure; thus, only the MBP need be recorded thereafter.