The purpose of this study is to demonstrate the antihypertensive efficacy of once daily oral administration of ACT 280778 on DBP compared to placebo after 28 days.

Inclusion Criteria:

• Males and females aged 18 to 75 years (inclusive) at screening.
• Body mass index (BMI) between 18 and 35 kg/m2 (inclusive) and body weight at least 50 kg at screening and prior to enrollment.
• 12-lead ECG without clinically relevant abnormalities measured at screening.
• Clinical chemistry, hematology, coagulation, virus serology, and urinalysis test results not deviating to a clinically relevant extent from the normal range at screening.
• Signed informed consent in the local language prior to any study-mandated procedure

Exclusion Criteria:

• Mean SBP > 180 mmHg.
• Severe, malignant, or secondary hypertension.
• Episodes of hypertensive crisis or hypertensive emergency within 6 months prior to enrollment.
• Previous history of fainting, collapse, syncope, orthostatic hypotension, or vasovagal reactions considered to be of clinical significance.
• E6 Severe coronary artery disease indicated by myocardial infarction, percutaneous coronary intervention, or coronary artery bypass graft within the last 12 months prior to enrollment.
• Angina pectoris within 6 months prior to enrollment