Twenty-four hour ambulatory blood pressure and electrocardiographic monitoring (Oxford method) was carried out in baseline conditions and after 15 days of therapy with a once a day administration of 240 mg verapamil slow-release in 11 elderly hypertensive patients. The 24 hour electrocardiographic monitoring (but not the blood pressure monitoring) was also carried out after 30 days of therapy. High-pressure liquid chromatography plasma concentrations of verapamil and its metabolite norverapamil were compared with the blood pressure and electrocardiographic responses. Systolic and diastolic blood pressure were significantly reduced after 15 days of therapy without circadian rhythm modification; mean heart rate was reduced after 15 and 30 days and the P-Q interval prolonged. Peak verapamil and norverapamil plasma concentrations were observed 8 hours after administration; at the 25th hour, the concentrations were respectively 60.9% and 68.3% of peak value (139.5 +/- 95.4 ng/ml and 126.4 +/- 60.9 ng/ml). Plasma levels of verapamil and norverapamil significantly correlated with heart-rate decrease. No correlation was observed between drug concentrations and systolic and diastolic blood pressure. A first-degree atrioventricular block was observed in 3 patients during therapy. Supraventricular premature contractions showed a decrease after 15 and 30 days of therapy. Thus, once a day administration of verapamil slow release is an effective anti-hypertensive medication in elderly hypertensive patients. Caution should be exercised in patients with P-Q interval prolongation and sinus bradycardia.