Cilazapril (CLZ) is a new, long-acting nonsulfhydril converting enzyme inhibitor (ACE-I). Its effect on peak and trough sitting diastolic blood pressure (SDBP) was studied in a total of 85 patients with uncomplicated, essential hypertension at three centers. After 4 weeks of a single-blind placebo (PLA) run-in period, patients whose SDBP was between 100 and 115 mm Hg, were randomized into active treatment with either PLA (n = 27), CLZ 2.5 mg (n = 29), or CLZ 5 mg (n = 29) once daily in a double-blind fashion for another 8 weeks. At the end of the PLA run-in and after 4 and 8 weeks active therapy, an hourly blood pressure (BP) profile during 1-10 and 21-24 h postdose was performed. The drop in SDBP at the end of the active treatment period at peak and trough was statistically and clinically significant for both CLZ doses in comparison with the PLA group. The peak/trough ratio after subtraction of the PLA effect was 62% for CLZ 2.5 mg and 59% for CLZ 5 mg. These results indicate that the dose regimen of CLZ 2.5-5 mg once daily is adequate and effective for 24 h.