Hypothermia and rewarming are associated with an increased incidence of lethal arrhythmias in man. The relationship between reduction in body temperature and ventricular fibrillation threshold was studied in 7 pentobarbital anaesthetized dogs using programmable electrical stimulation while cooling and rewarming between 37 degrees C and 25 degrees C in steps of 3 degrees C. Fibrillation threshold was defined as the number of extrastimuli required to evoke ventricular fibrillation. QRS-durations and corrected QT-intervals (QTc) were measured from surface electrocardiograms. Monophasic action potential durations were recorded from the base and apex of the heart. Fibrillation threshold decreased with decreasing temperatures; e.g., at 37 degrees C ventricular fibrillation was not inducible after 5 extrastimuli, while at 25 degrees C only 2 extrastimuli were required. From 37 degrees C to 25 degrees C QRS-width, monophasic action potential durations and QTc increased while conduction velocity decreased. The differential effects on conduction and monophasic action potential duration provide a basis for induction of ventricular fibrillation during acute hypothermia. This model of hypothermia-induced ventricular fibrillation should prove useful for future studies aimed at understanding the mechanisms responsible for hypothermia-related deaths.