Twenty-one patients with arrhythmias responsive to quinidine were studied both with regard to consistency of response as determined by repeat Holter Monitoring and in half of these patients a comparison of short and long-acting quinidine was made. Holter Monitoring demonstrated greater than 60% reduc tion of ectopic activity in all but one patient and reproducibility varied by less than 10% in nineteen of twenty-one patients. Nine of eleven patients showed the same response to long-acting quinidine as compared to short-acting. Eight of nine patients having significant numbers of ectopic pairs showed a significant reduction with quinidine therapy, both short-acting and long-acting.