To assess the duration of the antihypertensive effect of the dihydropiridine calcium antagonist felodipine in conventional (C-F) and slow-release (ER-F) formulations, 12 patients with essential hypertension underwent ambulatory blood pressure monitoring (ABPM) at the end of a 2-week treatment period with C-F 5 mg b.d., ER-F 10 mg once daily (o.d.) and placebo. C-F, ER-F and placebo were given in a double-blind 3 x 3 latin square design 4 times replicated. There was no systematic change in the ABP profile over the three study periods regardless of the treatment. In comparison to placebo, the mean 24-h systolic and diastolic blood pressures showed a significant and similar reduction after both formulations of F. Compared to placebo, C-F and ER-F induced a significant reduction in systolic blood pressure for 15 and 21 h, respectively, and of diastolic blood pressure for 16 and 21 h, respectively. Three patients complained of headache (mild in 2, moderately severe in 1), and two patients of nocturia, with either formulation of F.