OBJECTIVE Computerized analysis of blood pressure in conscious freely moving rats is a sound technique for physiological 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. 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 intragastrically via the gastric catheter. (2) Modified probability sum test was used to evaluate the synergism of two drugs. The formula was: q=P(A+B)/(PA+PB-PAxB). 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. CONCLUSIONS The two methods introduced by the present work will be important and useful for antihypertensive drug evaluation in conscious freely moving rats.