The efficacy on premature ventricular contractions (PVCs) of tocainide 600 mg three times daily and of quinidine sulfate 300 mg four times daily administered for 8 wk versus placebo administered for 6 wk was determined in 41 patients. Neither the subjects nor the interpreter of the Holter recordings knew which medication had been used. Adverse effects occurred in 14 of 22 patients (64%) on tocainide and in nine of 19 (47%) on quinidine. Adverse effects caused 13 of 22 patients (59%) on tocainide and six of 19 (32%) on quinidine to discontinue medication. Reduction of PVCs greater than or equal to 75% over placebo occurred in one of nine patients (11%) on tocainide and in six of 13 (46%) on quinidine. By Lown's classification, a reduction in one grade of PVCs occured in two of nine patients (22%) on tocainide and in eight of 13 (62%) on quinidine. Neither tocainide nor quinidine has a high incidence of efficacy and a low incidence of adverse effects but, in the doses used, quinidine was more effective, benefited a larger proportion of patients, and induced fewer adverse effects than tocainide.