1 The pharmacokinetics of two different sustained‐release oral procainamide preparations were studied in ten hospital patients with normal blood ureas and no clinical evidence of heart failure. Each patient received either one or other preparation at 12 hourly intervals for four doses. Frequent blood sampling enabled close monitoring of blood levels. 2 Results showed that both preparations were essentially similar in their pharmacokinetics. Both effectively double the half‐ life of conventional oral procainamide to 6.5 h and are suitable as prophylactic preparations. One patient developed toxic levels, thought to be related to her metabolic status of being a very slow acetylator. To avoid toxicity pre‐therapy assessment of a patient's cardiac and renal function and acetylator status is advised.