Ten patients with various heart diseases and ventricular arrhythmia received a single oral dose of disopyramide (DE) 200 mg. The ECG was recorded continuously for about 50 h from 2-4 h before drug administration. A statistically significant reduction in the number of ventricular ectopic beats (VEBs) was seen 1.0-3.5 h after drug intake; the average number of VEBs per 30 min decreased from 317 during the control period to 92 by 1.0-3.5 h after treatment and if one patient who did not respond is excluded, the corresponding figures were 272 and 14, respectively. Consecutive VEBs were seen in seven patients before DE was given and decreased significantly (p less than 0.05) 1.5-5.5 h after drug administration. There was no change in the PQ interval, the QRS interval showed a slight increase, whereas the QT interval was prolonged 0.5-4 h after administration of DE. A specific gas chromatographic method was used for DE assay in plasma and urine. Absorption was rapid in all patients. Urinary excretion during the first 48 h after drug intake varied between 35 and 75%. The lowest effective antiarrhythmic concentration estimated in six patients ranged from 1.4 to 7.0 mug/ml. beta-Phase half-life in five patients was between 10.3 and 22.1 h.