The aim of this study was to evaluate the usefulness of the angiotensin converting enzyme (ACE) inhibitor enalapril in a group of 30 patients (mean age 73.3 years) with moderate hypertension and normal haematological and chemical parameters (170 +/- 8.1 mmHg systolic and 104 +/- 5.8 mmHg diastolic blood pressure), who were receiving diuretic therapy with chlorthalidone (12.5 mg/day). This therapy caused a significant decrease in systolic and diastolic blood pressure (to 165 +/- 6.7 and 98 +/- 4.7 mmHg, respectively; P less than 0.001) but it also induced hypokalaemia (3.04 +/- 0.7 mmol/l; P less than 0.001) and multiple (greater than 10/h) and complex premature ventricular depolarizations (2nd, 3rd and 4th Lown grade). Enalapril treatment (5 mg/day for 5 days and 10 mg thereafter) was added to the diuretic therapy and after 2 months a further decrease in blood pressure was observed (to 158 +/- 5.6 mmHg systolic, P less than 0.001; 87.2 +/- 5.0 mmHg diastolic, P less than 0.001). Moreover, there was a significant reduction in the mean heart rate (from 79 to 72 beats/min, P less than 0.005) and an increase in serum potassium (to 4.19 +/- 0.2 mmol/l; P less than 0.001). In 80% of patients a 24-h dynamic electrocardiogram showed a significant reduction in both the number and complexity of premature ventricular depolarizations. Our findings suggest that ACE inhibitors can be useful in patients developing hypokalaemia during therapy. However, we are not yet able to explain the beneficial effects of enalapril in decreasing the frequency of premature ventricular depolarizations.