A study was carried out in 33 patients with myocardial infarction and complicating arrhythmias to compare plasma levels of disopyramide attained after administration of conventional capsules or sustained-release tablets. On Day 1, 29 patients started treatment wtih 600 mg disopyramide in 3 divided doses of conventional capsules. In 18, disopyramide plasma concentrations of 3 mg/l or more were achieved within 300 minutes after the first dose. On the second day, patients were allocated at random to receive treatment double-blind for 11 days with 500 mg disopyramide daily given either as conventional capsules (17 patients) or as sustained-release tablets (16 patients). Plasma concentrations were measured just before and 3 hours after the morning dose. Mean maximum and minimum concentrations in the conventional capsule group were 4.4 mg/l and 2.8 mg/l, respectively, compared to 4.3 mg/l and 3.0 mg/l, respectively, in the sustained-release tablet group. For the next 30 days all patients continued treatment with sustained-release tablets. Mean disopyramide plasma concentration measured 15 to 18 hours after the last dose was 2.8 mg/l. Myocardial function was estimated during and 1 week after discontinuing disopyramide treatment. The mean fractional fibre shortening (echocardiography) during treatment was 23% less than that 1 week after discontinuation. The mean PEP/LVET (systolic time interval recording) was 7% higher during treatment compared with 1 week after discontinuation. In 4 cases, mean ejection fraction, measured by scintigraphic methods, during treatment was 10% less than 1 week after discontinuation. Anti-arrhythmic effect was investigated by continuous monitoring in the first 12 days and by Holter monitoring during the 30-day ambulant period and 1 week after discontinuing disopyramide. A significant reappearance of warning arrhythmias could be detected after discontinuing disopyramide sustained-release tablets. No anti-arrhythmic effect was detected in 4 patients. Six patients had to be withdrawn because of side-effects. It is concluded that disopyramide sustained-release tablets 500 mg per day in 2 divided doses gives therapeutic plasma concentrations of 3 mg/l comparable with conventional capsules. A myocardial depressive effect is noted of about 10%.