To evaluate the magnitude and duration of the antihypertensive effect of sustained release (SRO) isradipine, 37 uncomplicated essential hypertensive patients (diastolic blood pressure 100-115 mm Hg after a one month run-in on placebo) were randomised to receive, according to a double-blind cross-over design, isradipine SRO 5 mg once daily and the corresponding placebo for 1 month. At the end of each treatment period, sitting blood pressure and heart rate were measured immediately before and every hour for 6 h after the last dose. Thirty-four patients [16 m, age 54 (7) y] completed the study. As compared to randomised placebo, isradipine SRO significantly reduced the systolic (SBP) and diastolic (DBP) blood pressure. Absolute DBP decrements versus placebo peaked 6 h after dosing (-8.8 mm Hg) and were not significantly lower (-8.2 mm Hg) at the end of the dose interval. At the same times, the absolute decrements in SBP were -9.8 mm Hg and -9.7 mm Hg, respectively. DBP was normalised in 19 patients (56%) at peak and in 17 (50%) at trough time. The trough to peak efficacy ratio in patients with peak DBP < or = 90 mm Hg was 70%. Heart rate was slightly increased by isradipine SRO. Adverse effects monitored with a check-list occurred in 8/36 patients (22%) on isradipine SRO and in 4/35 (11%) on randomized placebo. The data suggest that isradipine SRO is an effective antihypertensive drug, with a trough to peak efficacy ratio supporting once daily administration in most mild to moderate essential hypertensives.