The ECG and haemodynamic alterations caused by the i.v. infusion of platelet-activating factor (PAF) in the rabbit were studied after pretreatment with Kadsurenone, a specific PAF-receptor antagonist. Infusion of PAF at 0.8 microgram/kg in the rabbit caused important ECG changes, such as ST-segment depression and conduction arrhythmias, concomitantly with a marked reduction in left ventricular systolic pressure, mean arterial pressure and cardiac output, with a rise in total peripheral resistances and mean right atrial pressure. These physiological parameters became maximal at 75-120 seconds after PAF challenge, and returned to near prechallenge values within 25-60 minutes. Pretreatment with Kadsurenone, administered either intravenously (0.014 M, 1 ml/kg) or intraperitoneally (0.14 M, 1 ml/kg), exerted a quite complete protective effect in regard to the ECG changes and caused a significant reduction in the magnitude of all the haemodynamic alterations observed after intravenous infusion of PAF (0.8 microgram/kg). These results suggest that Kadsurenone is an effective inhibitor of PAF-induced cardiovascular changes in the rabbit, probably due to its competitive antagonism against PAF binding to specific receptors.