Noninvasive automatic ambulatory blood pressure monitoring during 24 hours in eight patients with moderate hypertension was used to determine the blood pressure response to lisinopril, an angiotensin-converting enzyme inhibitor. Office, 24-hour ambulatory, awake ambulatory, and sleep ambulatory diastolic blood pressures were decreased from 108 +/- 3, 98 +/- 8, 101 +/- 7, and 87 +/- 14 mm Hg, respectively, at baseline to 83 +/- 4 (P less than or equal to 0.0001), 82 +/- 7 (P less than 0.0001), 84 +/- 7 (P less than 0.0001), and 73 +/- 9 mm Hg (P less than 0.005), respectively, after 20 weeks of lisinopril treatment (dose range, 40 to 80 mg once daily). The diastolic blood pressure loads (percentages of ambulatory diastolic blood pressures more than 90 mm Hg) during 24 hours and during awake hours were 74% +/- 19% and 83% +/- 15%, respectively, at baseline and 24% +/- 19% (P less than 0.0001) and 29% +/- 21% (P less than 0.0001), respectively, during treatment. Heart rate was not altered by lisinopril. In conclusion, lisinopril is an effective antihypertensive agent for the treatment of moderate hypertension, and ambulatory blood pressures and diastolic blood pressure loads are useful for evaluating therapy for hypertension.