An open study was undertaken to investigate the efficacy and adverse effects of indoramin in 33 patients with essential hypertension whose blood pressure was uncontrolled (diastolic blood pressure 96 to 115 mm Hg) despite previous treatment with one or two antihypertensive drugs. Indoramin was added to the existing antihypertensive therapy and the dose titrated to a maximum of 150 mg/day or until blood pressure control was achieved (diastolic blood pressure less than 90 mm Hg or a reduction in diastolic blood pressure of 15 mm Hg). Patients were then followed up for a further 4 weeks. Indoramin significantly reduced mean standing systolic and diastolic blood pressure from 167/113 +/- 19.8/7.2 (SD) mm Hg to 150.3/101.1 +/- 23.4/8.9 (SD) mm Hg after 10 weeks and mean supine systolic and diastolic blood pressure from 169.8/110.8 +/- 16.4/5.6 (SD) mm Hg to 154.2/102.1 +/- 23.8/12 (SD) mm Hg after 10 weeks. Blood pressure was controlled in 21 of the 33 patients (63.6%) studied. Indoramin caused a small but significant fall in pulse rate of 3.9 beats per minute, in the supine position, after 4 weeks therapy. 10 patients experienced adverse effects, the most common being dizziness and headache (3 patients each), and lightheadedness/fainting on standing (2 patients). No patient experienced sedation. Only 1 patient was withdrawn from the trial because of adverse effects (fainting on standing). Biochemical and haematological investigations carried out pretreatment and during treatment showed no abnormalities related to indoramin treatment.