The tail-cuff methods for measuring systolic blood pressure in the rat usually require preheating of the animal to obtain recordable pulse signals. To find a more sensitive method, we applied the principle of differentiated impedance (dZ/dt) to the tail-cuff measurement of systolic blood pressure. We obtained clear pulse signals from the tail in awake rats without preheating the animals, and the systolic blood pressure obtained by this method had an excellent correlation with the directly measured femoral artery pressure (correlation coefficient=0.98). Heating the animals at 40°C for 5 minutes increased systolic blood pressure by a mean of 6 mm Hg as compared with that determined at the ambient temperature of 21 to 24°C. Mean systolic blood pressure in young female diabetic rats was 122 ± 3 mm Hg, which was significantly higher than the 111 ± 2 mm Hg of normal rats. It is concluded that the technique of electrical impedance as applied to the tail-cuff method is simple and highly sensitive and is suitable for measurement of tail systolic blood pressure in awake rats without preheating.