The efficacy and safety of a new slow-release formulation of nifedipine ("Adalat Retard") were assessed in a double-blind cross-over trial in 19 subjects with essential hypertension (14 male, 5 female--ages: 34-72 years), 14 of whom continued previous antihypertensive medication. There were two 6 week treatment phases in which nifedipine 20 mg twice daily and placebo tablets twice daily were administered in random order. Supine mean blood pressure was 115 +/- 2 mm Hg during the placebo phase and 105 +/- 2 mm Hg during the nifedipine phase (p less than 0.001); and standing mean blood pressure was 121 +/- 2 mm Hg after placebo and 110 +/- 2 mm Hg after nifedipine (p less than 0.001). The magnitude of the blood pressure difference between the two phases was not related either to age or to the placebo phase blood pressure. The hypotensive effect of nifedipine was observed when administered as a single agent or in combination with diuretic and/or beta blocker. Heart rate was increased after nifedipine--75 +/- 2 beats/minute compared with 71 +/- 2 beats/minute after placebo (p less than 0.01). In this dose nifedipine (as "Adalat Retard") is an effective hypotensive agent which is a useful addition to presently available therapy.