Factors which may predispose to ventricular fibrillation were assessed in 48 consecutive patients who developed ventricular fibrillation outside hospital after the arrival of a mobile coronary care team. Before the initiation of ventricular fibrillation, late cycle ventricular extrasystoles were recorded in 38 patients (79%) and R on T extrasystoles in 27 patients (56%). R on T extrasystoles were relatively infrequent--an average of three occurred during a mean monitoring time of 27 minutes. Multifocal extrasystoles occurred in only three patients (6%), consecutive extrasystoles in 14 (29%), and self-terminating ventricular tachycardia in three (6%). Ventricular extrasystoles occurring at more than five a minute were uncommon during this phase. The time from "warning arrhythmias" to the development of ventricular fibrillation in many patients was short, thus limiting the administration of antiarrhythmic agents. There was a significant increase in the heart rates recorded immediately before ventricular fibrillation when compared with those documented initially. Thus, an increase in heart rate appeared to be a predisposing factor in the initiation of ventricular fibrillation. An R on T extrasystole was the most important factor in the initiation of ventricular fibrillation: it occurred in 33 patients (69%). In nine (19%) ventricular tachycardia and in six (12%) a late cycle extrasystole or idioventricular rhythm initiated ventricular fibrillation.