The acute haemodynamic effect of metoprolol was investigated in 14 patients with essential hypertension (7 of the WHO stage I, 7 of stage II). The evaluated parameters include the mean arterial blood pressure (MBP), heart rate (HR), cardiac index (CI), total peripheral resistance (TPR) and capillary blood flow of the forearm muscle (CBF). Investigation was carried out within two days: the 1st day at rest and after an infusion of 20% mannitol, the 2nd day with additional previous intravenous administration of 5 mg of metoprolol. The findings were as follows: 1. There was a decrease of MBP, a slowdown of HR and a decrease of CBF after metoprolol both in hypertensives I and II. 2. The different haemodynamic response of the Ist and IInd stages of essential hypertension was manifested by a significant decrease of CI and an increase of TPR in hypertensives II (but not I) not only after the infusion of mannitol alone, but also after i.v. administration of metoprolol. The exaggerated haemodynamic response to acute metoprolol administration in essential hypertension II could be caused by latently impaired cardiac performance in this stage.