The purpose of this study is to determine the response rate for the therapeutic goal (diastolic blood pressure (BP) less than 90 mmHg and systolic BP less than 140 mmHg) at Week 6 for quinapril 20 mg and at Week 12 for quinapril 20 mg, quinapril 40 mg, and quinapril 20 mg plus hydrochlorothiazide 12.5 mg; and to determine the mean decrease in diastolic and systolic BP measurements and the change in heart rate at Week 6 and Week 12 compared with baseline.

Inclusion Criteria:

• A clinical diagnosis of mild to moderate primary hypertension ("Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure, 6th report" Stage I or II)
• Sitting diastolic blood pressure between 90 mmHg and 109 mmHg and systolic blood pressure between 140 mmHg and 179 mmHg at the end of the no-treatment run-in period

Exclusion Criteria:

• Subjects with secondary hypertension
• Subjects with serious cardiac disease (including myocardial infarction in the last 3 months or decompensated heart failure)
• Subjects who do not accept to stop using any other antihypertensive treatment during the run-in period and the treatment period of the study