In a randomized placebo-controlled double-blind trial, we compared flecainide to quinidine for treatment of ventricular ectopic depolarizations in 19 patients. The mean percent suppression of total ventricular ectopic depolarizations was 95% for flecainide and 56% for quinidine (p < 0.05). A greater than 80% reduction of total ventricular ectopic depolarizations was obtained in eight of nine patients given flecainide and in five of ten patients given quinidine (p = 0.09). After the randomized protocol, the patients who had received quinidine were given flecainide; 9 of the 10 patients had greater than 80% reduction of total ventricular ectopic depolarizations. Flecainide produced 100% suppression of nonsustained ventricular tachycardia and 99.5% suppression of paired ventricular depolarizations. Flecainide prolonged the PR and QRS intervals; quinidine prolonged the PR and JT(C) intervals. Side effects were commoner with quinidine than flecainide (p = 0.06). Three other patients failed to complete the protocol because of serious adverse experiences.