Contrary to their reputation as being inert, aerosol fluoroalkane propellant gases (Freons) are rapidly acting and potent cardiac toxins. This discovery, first made in mice, has been confirmed during adequate oxygenation, either in vitro or in vivo, in rats, cats, dogs, monkeys, and man. This brief review demonstrates that propellant gases are toxic to the mouse heart, enter the blood after inhalation, and, despite adequate oxygenation, quickly cause ventricular tachyarrhythmias in monkeys and lower arterial pressure and peripheral resistance in several species, and are directly toxic to ventricular myocardium, profoundly depressing its contractility in all species studied, including man. Ventricular tachyarrhythmias, bradyarrhythmias, acute heart failure, arterial hypotension, and asphyxia may cause sudden death in youths who deliberately inhale aerosol propellants. The possibility that these gases are harmful, acutely or chronically, to frequent aerosol users requires further study.