A double-blind, placebo-controlled, cross-over study was carried out to evaluate the efficacy and safety of 2.5 mg indapamide in 24 hypertensive patients failing to respond to oxprenolol alone. An additional 6 patients were assessed by ambulatory blood pressure recordings over a 15-hour period with a Remler M2,000 semi-automatic sphygmomanometer. On average, indapamide reduced supine blood pressure by 18.5/10 mmHg and standing blood pressure by 19.6/8.9 mmHg. The ambulatory recordings carried out in 6 patients detected a fall in diastolic pressure not observed using clinic readings in these 6 patients, suggesting that this is a more sensitive method of detecting antihypertensive effect. These responses were not associated with significant changes in heart rate or body weight and there was no significant postural fall in blood pressure. No serious side-effects were reported. Changes in serum potassium, chloride and urate similar to those seen with diuretics were observed. These results suggest that indapamide is a useful and safe adjunct to beta-adrenoceptor blocking therapy for uncontrolled hypertension.