A total of 190 patients (mean age 57 years; range 21 to 89 years), with a supine diastolic blood pressure (sDBP) of 100 to 120 mm Hg after a 3-week placebo phase, entered a double-blind parallel-group study to receive placebo (n = 62), or slow-release isradipine (SRO) at 2.5 mg (n = 64) or 5 mg (n = 64) once daily for 4 weeks. Blood pressure was always measured between 23.5 and 24.5 h after the previous drug administration. Before and after the 4-week treatment period, blood pressure profiles were recorded from measurements taken immediately before drug administration ('trough') and repeated at 2, 4, 6, and 8 h after administration. A 'peak:trough' ratio was assessed from the data of 156 patients who completed the 4 weeks of active treatment, and for whom blood pressure profiles were available both before and after treatment. A supine DBP of < or = 90 mm Hg at trough was achieved by 28%, 35%, and 47% of patients receiving placebo, 2.5 mg, and 5 mg SRO, respectively. If patients who achieved a reduction in sDBP of at least 10 mm Hg are included, the total response rate becomes 42%, 66%, and 61% in the three treatment groups, respectively. The reduction in sDBP immediately before drug administration (trough effect) was 37% of the 'peak' effect (6 h after administration) with the 2.5-mg dose and 74% with the 5-mg dose. Adverse events were reported by 14.5%, 9.4%, and 17.2% of patients taking placebo and SRO at 2.5 mg and 5 mg, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)