Antihypertensive and diuretic actions of indapamide (SE-1520) were investigated in rats and compared with those of trichlormethiazide (TCMT). In normotensive rats, indapamide in a dose of 100 mg/kg p.o. did not show a significant effect on the blood pressure. In DOCA-saline and uni-nephrectomized DOCA-saline hypertensive rats, indapamide above 1 mg/kg and TCMT above 3 mg/kg with single oral or repetitive administration for 2 weeks reduced the blood pressure level. In spontaneously hypertensive rats (SHR), both indapamide and TCMT lowered the blood pressure with single doses above 10 mg/kg or repetitive doses above 3 and 10 mg/kg, respectively. In the diuretic test using normal rats, indapamide in doses ranging from 0.1 to 30 mg/kg increased urine volume and urinary electrolyte excretion. TCMT showed a more potent diuretic action at a lower dose level. In SHR, indapamide and TCMT produced a greater urine volume and electrolytes excretion. Indapamide inhibited the carbonic anhydrase activity and the potency was about 1/25 of that of acetazolamide in vitro. The antihypertensive activity of indapamide was more potent than that of TCMT and the reverse order to that of their diuretic potencies. It is suggested that the mechanism of antihypertensive effect of indapamide is different from that of TCMT.