Serum quinidine concentrations are often helpful in aiding the physician in the most effective use of quinidine. Knowledge of the serum level allows more rational treatment of both acute and chronic arrhythmias, and permits a clearer awareness of the possibility of toxicity. The physician who appreciates the quantitative aspects of the use of quinidine will be in a better position to weigh the relative risks of the drug against the potential benefits to be gained. Control of therapy through knowledge of the serum quinidine concentration should decrease the risk of toxicity to the patient and permit safer and more effective therapy.