Circadian rhythms of blood pressure, heart rate, and locomotor activity were measured with implanted radio-telemetry transmitters in conscious, unrestrained spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). During the light period (0600 to 1800 h) systolic, diastolic, and mean blood pressure, heart rate, and locomotor activity were lower than during the dark period (1800 to 0600 h). During the first and the last hour of the dark period, all parameters showed a relative peak when compared to values during the rest of the night. The pattern of circadian changes in blood pressure, heart rate, or behavior was essentially similar in SHR and WKY. Blood pressure values were always higher in SHR than in WKY, but there was no general strain difference in heart rate or locomotor activity values. The administration of hydralazine in the drinking water resulted in a significant decrease in blood pressure in SHR and WKY, and a suppression of the difference between light-phase values and dark-phase values. By contrast, heart rate showed an overall increase in both strains, whereas locomotor activity was largely unaffected. These data show the validity of telemetry as a means of measuring circadian rhythms of blood pressure, heart rate, and behavior in freely moving rats. Apart from their characteristic hypertension, SHR show few differences with WKY with respect to other parameters, such as circadian rhythmicity, baseline heart rate values, or behavior. Some of the previously published differences between SHR and WKY may have been influenced by the stress of the experimental procedures used.