Nineteen patients (12 male and seven female) with essential hypertension (mean arterial pressure ranging from 113-162 mmHg) were studied at rest, supine and sitting, and during bicycle exercise (50, 100 and 150 W). Intra-arterial blood pressure and heart rate were recorded continuously. Cardiac output was measured by dye dilution (Cardiogreen). The patients were treated with enalapril 10-40 mg (mean 33 mg) once-daily; seven patients received in addition 25-50 mg hydrochlorothiazide daily. After 5-13 months (mean 10 months) patients were restudied. Blood pressure fell (P less than 0.001) in all - at rest sitting from 184/108 to 150/89 mmHg (-19%) and during 100 W bicycle exercise from 225/118 to 197/101 mmHg (-13%). Pretreatment total peripheral resistance index (TPRI) was markedly increased and fell at rest sitting from 4087 to 3514 dyn s/cm-5 m2 (-14%) (P less than 0.05). No significant change was seen in cardiac output, heart rate or stroke volume. Overall body weight and body fluid volume (isotope dilution technique) remained unchanged, but in the subgroup receiving enalapril + hydrochlorothiazide, blood volume fell by 12% (P less than 0.01). No side-effects were seen. In conclusion, enalapril monotherapy reduces blood pressure in two-thirds of patients with moderately severe essential hypertension at rest and during exercise, associated with a significant reduction in TPRI. One-third of patients require a diuretic in addition to enalapril. In this subgroup the reduction in TPRI was about twice the reduction seen during enalapril treatment alone, and there was a fall in blood volume.