Ten patients (mean age 53 years) with essential hypertension have been studied at rest and during exercise following oral treatment for 6 weeks with a new beta-adrenoceptor blocking agent, bopindolol. The treatment caused a significant decrease in systolic and diastolic arterial blood pressure and heart rate, both at rest and during exercise. Stroke volume fell, too, and therefore so did cardiac output, whereas the systemic vascular resistance was increased. Left ventricular filling pressure was elevated both at rest and during exercise following bopindolol therapy. However, a different haemodynamic pattern was noted in patients with elevated total peripheral resistance prior to therapy (Group 1) compared to patients with normal or subnormal peripheral resistance (Group 2). A decrease in systemic vascular resistance seemed to be the cause of the fall in blood pressure in Group 1, as the expected increase in vascular resistance did not occur, whereas a reduction in cardiac output was of greater importance in Group 2. During exercise the lowering of arterial blood pressure in both groups was mediated by a reduction in cardiac output.