The antihypertensive effect of the angiotensin-converting enzyme (ACE) inhibitor lisinopril administered in a single dose of 20 mg was evaluated by ambulatory blood pressure monitoring (ABPM) in a double-blind, placebo-controlled, cross-over study. Twenty-four patients (21 men and 3 women, mean age 52 +/- 6 years) with mild to moderate hypertension were included in the study and randomly assigned to two consecutive treatments with lisinopril 20 mg and placebo, each administered for 4 weeks. On the last day of each treatment, BP was assessed by noninvasive 24-h ABPM. BP was significantly lower after lisinopril than after placebo in a 24-h period (mean 24-h systolic BP (SBP) with lisinopril 120 +/- 7 mm Hg and with placebo 135 +/- 9 mm Hg; mean day SBP with lisinopril 125 +/- 3 mm Hg and with placebo 142 +/- 5 mm Hg; mean night SBP with lisinopril 112 +/- 4 mm Hg and with placebo 124 +/- 6 mm Hg; mean 24-h diastolic BP (DBP) with lisinopril 76 +/- 6 mm Hg, and with placebo 87 +/- 8 mm Hg; mean day DBP with lisinopril 80 +/- 3 mm Hg and with placebo 93 +/- 4 mm Hg; mean night DBP with lisinopril 69 +/- 2 mm Hg and with placebo 79 +/- 5 mm Hg, p less than 0.001). Mean 24-h, mean day, and mean night heart rate (HR) did not differ significantly between placebo and lisinopril treatments. Repeated-measures analysis of variance (ANOVA) showed a significant influence on SBP (p less than 0.001) and DBP (p less than 0.001) throughout the treatment.(ABSTRACT TRUNCATED AT 250 WORDS)