30 patients with primary mild to moderate hypertension (DBP 95-110 mm Hg) were treated with long acting diltiazem (120 or 240 mg) once daily in the morning (7.00-8.00 a.m.) for at least 3 weeks and after that the administration time was changed to evening dose (19.00-20.00 p.m.) for next 3 weeks. 24-hours ABPM was performed in all patients on the last day of each period. Obtained recordings were compared in different periods of time: total 24 hours, 6.00-23.00, 23.00-6.00, 4.30-8.00, 5.00-11.00, 20.00-2.00. Mean values of systolic and diastolic blood pressure, heart rate and pressure load (defined as percentage of records above 140/90 mm Hg in day and above 120/80 mm Hg at night) did not differ significantly between investigated dosage regimens. The evening administration of diltiazem did not produce greater decrease of BP at night than the morning dose. For this reason slow release formulation of diltiazem (oxycardil in doses of 120 to 240 mg) can be safely administered in the evening if needed.