The purpose of the present study was to analyze the effect of voluntary exercise upon arterial blood pressure and heart rate, by means of direct measurement in unanesthetized,unrestrained Spontaneously Hypertensive Rats (SHR). 15 male SHR and Wistar Control Rats (WCR) aged 17-25 weeks old were used. After the animals were anesthetized, polyethylene catheter filled with heparinized saline was inserted into the femoral artery. The dwelling catheter was tunneled to the back of the neck, connected to pressure transducer via a rotating swievel. After recovering from anesthesia, each rat was housed individually in a rotating wheel capable of voluntary exercise. Three days (at least) after catheter implantation, continuous (24 hours/day) recording of arterial pressure, heart rate, and wheel rotation were initiated. The results were as follows: 1) Sufficient voluntary running exercise of SHR and WCR with dwelling catheter were observed during the continuous measurements. 2) Mean blood pressure at rest in SHR was 139 mmHg. The blood pressure significantly increased during voluntary exercise to 158 mmHg. In WCR, mean blood pressure at rest was 93.3mmHg, also significantly increased to 100 mmHg during voluntary exercise. 3) Heart rate in SHR at rest was 308 (beats/min). During exercise, the heart rate significantly increased from the resting level to 376 (beats/min). In WCR, heart rate also significantly increased from 335 (beats/min: at rest) to 423 (beats/min) during exercise. 4) The increase rate (v.s. resting level) of blood pressure during voluntary exercise of SHR (14%) was significantly larger than that of WCR (7.2%). 5) The increase rate (v.s. resting level) of heart rate during voluntary exercise of SHR (22%)was significantly smaller than that of WCR (26%). These results suggest that the intensity of voluntary running exercise of rat in rotating wheel is mild, and the increase response of blood pressure to the exercise is greater in SHR.