The ability of nitrendipine to control blood pressure (BP) over a 24-hour period was assessed in elderly patients over 65 years of age with a systolic BP (SBP) greater than 170 mmHg, and a diastolic BP (DBP) greater than 100-130 mmHg. Twenty-two patients were randomized equally to two groups: group 1 received the drug once daily and group 2, twice daily. The study was double-blind, with patients in group 1 receiving a matching placebo tablet instead of the second dose of nitrendipine before retiring to bed. Patients attended the hospital clinic at 10 a.m. before taking their morning tablet, so that BP was recorded 24 hours (group 1) or 12 hours (group 2) after nitrendipine. The study design permitted one dose titration if the target SBP of less than 170 mmHg or DBP of less than 100 mgHg was not achieved after three weeks of active therapy; the dose was doubled and the patients received 6 weeks of therapy at the preferred dosage. Mean DBP after 6 weeks' nitrendipine was comparable in the two groups and significantly lower than that with placebo (P less than 0.001), but mean systolic BP was lowered only in group 2 patients (P less than 0.001). There was no significant difference in mean SBP or DBP between patients on 20 mg daily and 10 mg b.d. Therefore, in elderly patients, once daily nitrendipine controls DBP as satisfactorily as the same daily dose given in two divided doses, but does not reduce SBP as adequately.