A study was carried out to evaluate the efficacy of indapamide in the treatment of 20 patients with mild hypertension and to determine whether a favourable response to treatment was related to initial hyperadrenergic status, to changes in serum catecholamine levels or modified by orthostatic stress. After 4 weeks on placebo, patients received 2.5 mg indapamide per day for 4 weeks and, if diastolic pressure was controlled below 100 mmHg, this regimen was continued for a further 8 weeks. Those not adequately controlled with indapamide alone were treated additionally with nadolol (mean dose 140 mg per day) for a period of 12 weeks. The results showed that indapamide alone produced a significant reduction in blood pressure both in the recumbent and upright position in 12 (60%) of the patients and that addition of the beta-blocker augmented the antihypertensive effect. No relationship was demonstrated between initial high serum catecholamine levels and a favourable response to indapamide. Recumbent serum norepinephrine levels after indapamide alone or with nadolol for 4 to 8 weeks remained comparable with placebo levels. However; the amplitude of the norepinephrine increase due to orthostatic stress was significantly reduced after 8 weeks of indapamide treatment. Possible explanations for this are discussed.