The objective of the study was to examine the influence of angiotensin converting enzyme (ACE) inhibition on circulatory responses to standardized stress tests in primary mild to moderate hypertension. Patients (n = 28) received 5 mg ramipril daily or placebo for 6 weeks in a double-blind crossover design, followed by 6 months of open ramipril treatment. Mental stress (a 20-min Stroop's color word conflict test) and a cold pressor test were performed at the end of each of the three study periods. Noninvasive blood pressure and heart rate were recorded. Ramipril reduced systolic and diastolic blood pressure levels at rest (from 146+/-3/99+/-3 with placebo to 135+/-4/94+/-3 at 6 weeks, and 136+/-4/91+/-3 mm Hg at 6 months, in the laboratory) and during mental stress. Resting heart rates were unchanged by ramipril. Ramipril reduced systolic blood pressure and heart rate responses during mental stress; diastolic blood pressure responses were unchanged. Ramipril reduced cardiac workload (systolic blood pressure x heart rate) levels and responses. Treatment effects at 6 months were generally greater than at 6 weeks. During the cold pressor test systolic and diastolic blood pressure levels were lowered by ramipril, but responses were unchanged. Heart rate responses, however, were reduced. Thus, ramipril reduced cardiac workload levels and responses also during the cold pressor test. These findings show that ACE inhibitors can reduce cardiac workload during stressful situations. If confirmed, this would seem to offer an advantage in the treatment of hypertension.