AIM: Computerized analysis of blood pressure in conscious freely moving rats is a sound technique for physiologi- cal and pharmacological studies. The present work, based on this technique, was designed to introduce two useful methods for the evaluation of antihypertensive drugs in conscious spontaneously hypertensive rat (SHR). They were the directly intragastric administration of drugs and modified probability sum test for evaluating the synergism of the combination of two drugs. METHODS AND RESULTS: (1) Directly intragastric administration was used in conscious rats. A catheter was inserted into stomach immediately after arterial catheter insertion. Three days after operation, blood pressure was recorded and drug might be given intragastically via the gastric catheter. (2) Modi- fied probability sum test was used to evaluate the synergism of two drugs. The formula was: q=PA+B/(PA+PB?PA×PB). With this method, it was obtained: q=1.32 for the effects of the combination of atenolol and nitrendipine (20 mg/kg+10 mg/kg) on systolic blood pressure; q=1.41 for the effects of the combination of atenolol and amlodipine (10 mg/ kg+1 mg/kg) on systolic blood pressure. CONCLUSION: The two methods introduced by the present work will be important and useful for antihypertensive drug evaluation in conscious freely moving rats. AIM: Computerized analysis of blood pressure in conscious freely moving rats is a sound technique for physiologi- cal and pharmacological studies. The present work, based on this technique, was designed to introduce two useful methods for the evaluation of antihypertensive drugs in conscious spontaneously hypertensive Rats (SHR). They were the directly intragastric administration of drugs and modified probability sum test for evaluating the synergism of the combination of two drugs. METHODS AND RESULTS: (1) Directly intragastric administration was used in conscious rats. A catheter was inserted into Three days after operation, blood pressure was recorded and drug might be given intragastically via the gastric catheter. (2) Modi-fied probability sum test was used to evaluate the synergism of two drugs. The formula was: q = PA + B / (PA + PB? PA × PB). With this method, it was obtained: q = 1.32 for the effects of the combination of aten ol and nitrendipine (20 mg / kg + 10 mg / kg) on ​​systolic blood pressure; q = 1.41 for the effects of the combination of atenolol and amlodipine (10 mg / kg + 1 mg / The two methods introduced by the present work will be important and useful for antihypertensive drug evaluation in conscious freely moving rats.