The cardiovascular effects of lysine acetylsalicylate and/or propranolol were studied in 26 dogs. All animals were maintained under anaesthesia with halothane 0.75 per cent, supplemented by the intravenous administration of succinylcholine to allow controlled ventilation during a two hour period of monitoring. Cardiac output, stroke volume, heart rate, mean arterial pressure, pulse pressure, central venous pressure, total peripheral resistance, pH, Paco2, pao2 and base deficit were measured in each dog. Lysine acetylsalicylate 50 mg . kg-1, administered alone as a single bolus, significantly (P less than 0.05) increased the cardiac output and stroke volume and significantly decreased the heart rate, central venous pressure and total peripheral resistance in dogs under halothane anaesthesia. Propranolol hydrochloride 0.5 mg . kg-1 as a single intravenous bolus was followed by a significant decrease in cardiac output, heart rate and mean arterial pressure and a significant increase in central venous pressure and total peripheral resistance. The administration of propranolol prior to lysine acetylsalicylate resulted in a significant decrease in cardiac output and heart rate. Pretreatment with propranolol was effective in inhibiting the positive inotropic effect of lysine acetylsalicylate.