The antihypertensive effect of clonidine administered concurrently with labetalol or metoprolol were studied with spontaneously hypertensive rats (SHR). The changes of heart rate were also observed in the same rats. Every drug was orally administered single dose after prechecking the systolic blood pressure and heart rate of SHR. The blood pressure of SHR in concurrently administered group was more significantly decreased than in alone administered group. The effective and stable decrease of blood pressure was maintained at the group of clonidine with labetalol (0.05+50mg/kg) for 9 hours. The group of clonidine with metoprolol (0.05+100mg/kg) manifested more marked decrease of blood pressure than the group of metoprolol (100mg/kg) alone for 9 hours. The diminishing effect of heart rate was enhanced in group of administering clonidine with labetalol, decreasing the dose of labetalol from 50mg/kg via 25mg/kg to 12.5mg/kg. On the other hand, in the group that clonidine was administered concurrently with metoprolol, the diminishing effect of heart rate was decreased with decreasing doses of metoprolol from 100mg/kg via 50mg/kg to 25mg/kg.