The blood levels in 20 patients were studied during the change from parenteral administration of disopyramide to an oral sustained release preparation containing the same active substance. From this study it is concluded that 100 mg i.v. bolus injection of disopyramide can safely be followed by the immediate administration of one sustained release tablet containing 250 mg disopyramide. In case of an i.v. infusion (0.4 mg kg‐1h‐1) it seems wise not to start earlier with the administration of the sustained release preparation than at the moment of stopping the infusion.