The antihypertensive effects of an angiotensin II antagonist, candesartan cilexetil (TCV-116), and other classes of antihypertensive drugs (including a calcium antagonist, manidipine; a diuretic, hydrochlorothiazide (HCTZ); an α-blocker, prazosin; and a β-blocker, atenolol) administered in combination were examined in spontaneously hypertensive rats by oral administration daily for 2wk. TCV-116 at 1mg/kg lowered the blood pressure by about 50 and 30mmHg, 5 and 24h after dosing, respectively. The blood pressure was slightly lowered by HCTZ at 10mg/kg, but it was synergistically reduced when HCTZ was given in combination with TCV-116. Manidipine at 3mg/kg lowered the blood pressure by about 50 mmHg 1h after administration. When manidipine was given in combination with TCV-116, blood pressure was reduced additively. Prazosin at 1mg/kg lowered the blood pressure by 40 to 50mmHg 1h after dosing. When prazosin was given in combination with TCV-116, the reduction was intensified more than additively, to about 100mmHg. Atenolol at 50mg/kg lowered the blood pressure by 10 to 20 mmHg 5h after dosing. Even when atenolol was administered in combination with TCV-116, the reduction in blood pressure was virtually the same as that observed when TCV-116 was given alone. TCV-116 and HCTZ had no effect on the pulse rate, whereas manidipine and prazosin both increased it, owing to reflex tachycardia, and atenolol decreased it. TCV-116 had no effect on the change in the pulse rate induced by these antihypertensive drugs and no effect on HCTZ-induced diuresis. TCV-116 and HCTZ each caused a significant increase in plasma renin concentration (PRC), and prazosin caused a slight elevation. Manidipine had no effect on the PRC, whereas atenolol reduced it. Given in combination with TCV-116, these antihypertensive drugs had the same effect on the elevated PRC induced by TCV-116 as they did on the basal PRC when administered alone. These results suggest that antihypertensive drugs that cause compensatory activation of the renin-angiotensin system have more marked antihypertensive activity when given in combination with TCV-116, but that there will is no combined effect on the pulse rate. (Hypertens Res 1996; 19: 247-254)