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To determine the clinic and ambulatory blood pressure when the same dose of perindopril (4 mg) is administered in the morning (0900 h), or at night (2100 h), in particular, to determine whether the early morning blood pressure rise, the duration of effect and the pattern of response differed.Twenty male patients with diastolic blood pressure 95–110 mmHg when seated and 24 h mean ambulatory diastolic blood pressure >85 mmHg after 4 weeks' placebo were allocated randomly to be administered 4 mg perindopril at 0900 h or at 2100 h. Clinic blood pressure (with the patient seated and erect) was measured 2 and 4 weeks after the therapy had been started and ambulatory blood pressure monitoring with a SpaceLabs device was performed for 26 h during week 4. The patients then crossed over to the other time of dosage and the measurements were repeated. The study was conducted from a hospital clinic.The clinic analysis concerned all 20 patients but the ambulatory analysis concerned 18 patients because the ambulatory blood pressure monitor data sets were inadequate for two patients. Compliance was high (97 ± 3%), with a suggestion that it was better with the 0900 h dose. Clinic blood pressure (with the patient seated and erect) was lower under both regimes and the blood pressure with night-time administration of perindopril tended to be lower than that with daytime administration (P = 0.05–0.10). Twenty-four-hour mean, daytime and night-time means were lower with both doses than they were with placebo and did not differ. Both regimes reduced the early morning peak blood pressure rise and the effect tended to be greater with the 2100 h dose (P = 0.05–0.10). The 0900 h dose had an effect that persisted for >24 h but the effect of the 2100 h dose had dissipated 18 h after the dose. There was no excessive night-time fall in blood pressure with the 2100 h dose. The trough: peak ratio with the 0900 h dose was 0.86 for systolic and 0.70 for diastolic blood pressure.The early morning blood pressure rise is reduced more when 4 mg perindopril is administered at 2100 h. However, the 2100 h dose regime does not reduce blood pressure over 24 h whereas 24 h control is achieved with the 0900 h dose. In clinical practice the 2100 h dose would have been titrated to the next dose range in more patients. This study indicates that the response profile obtained with an angiotensin converting enzyme inhibitor cannot be transformed from one dose time to another automatically and that chronobiology has important effects on a drug's action.