This one-year open study conducted in two centres investigated the 24-hour effectiveness of the ACE inhibitor perindopril in the treatment of 31 patients with mild or moderate hypertension. The investigation evaluated the so-called "through" effect, i.e. the effect of the drug at the end of the 24-hour dosage period. Monotherapy with perindopril (4 mg or 8 mg administered once a day) led to normalization of the diastolic pressure to values lower than 90 mmHg in 48% of the patients, and a combination of perindopril with hydrochlorothiazide produced this effect in another 38% of patients. Treatment thus was able to normalize the diastolic blood pressure in 86% of patients. The mean decline of systolic pressure in a recumbent position was 17.6 mmHg and 13 mmHg for diastolic pressure assessed in a recumbent position 24 hours after ingestion of the drug. The reduction of blood pressure achieved after three months treatment persisted for the whole year. In 93% of the patients the diastolic pressure was at the end of one year lower than 95 mmHg. The drug was very well tolerated and in none of the patients the treatment had to be discontinued because of undesirable side-effects. Perindopril is a significant contribution to the spectrum of drugs used in the treatment of hypertension because of its 24-hour effectiveness and very good tolerance.