Blood pressure, heart rate and physical activity in spontaneously hypertensive rats (SHR) were measured and compared with those of Wistar Kyoto rats (WKY) to see how these parameters vary with age and whether we can find a marker of essential hypertension among these variables. An automatic telemetric system was employed to measure these variables. In SHR, blood pressure and physical activity increased with age, but heart rate decreased. In WKY, neither blood pressure nor physical activity showed significant changes, but heart rate decreased with age. The 24h mean values of blood pressure, heart rate and physical activity were significantly higher in SHR than in WKY. The mean values of blood pressure, heart rate and physical activity were higher during nighttime than during daytime in both SHR and WKY. Circadian MESOR and amplitude of blood pressure increased simultaneously in SHR. These results are compatible with the concept that an increased amplitude of blood pressure may predict subsequent development of essential hypertension.