1. The effects of three beta-adrenoceptor blocking agents on isoprenaline induced changes in systemic and regional blood flow have been investigated in rats anaesthetized with pentobarbitone. The blood flow to the principal vascular beds was measured by the (86)Rb fractionation method.2. Isoprenaline infusion at a rate of 0.3 (mug/kg)/min intravenously caused an increase in cardiac output and a decrease in peripheral resistance which affected different vascular fields unevenly. The coronary and carcass fractions of cardiac output increased while the cutaneous, renal and splenic fractions decreased.3. Pretreatment with propranolol (2 mg/kg intravenously) blocked all isoprenaline effects. Practolol (8 mg/kg intravenously) lessened the effect on cardiac output but did not prevent the vascular effects except those on the coronary circulation. LB 46 (0.2 mg/kg) had less marked beta-adrenoceptor blocking activity than propranolol (2.0 mg/kg).