Plasma concentrations and bioavailability of disopyramide following repeated administration of standard capsules and controlled-release tablets have been compared. Ten patients were randomized into two groups; Group I received disopyramide capsules 150 mg every 6 h for five days. Group II received the same preparations in the reverse order. There was a more rapid rise in disopyramide concentration after the capsules: the maximum of 10.7 +/- 0.6 mumol/l (mean +/- SEM) was reached within 1.8 +/- 0.4 h as compared to 10.6 +/- 0.4 mumol/l within 4.0 +/- 0.3 h after the controlled-release tablets. No significant difference in the fluctuations in individual plasma concentrations during each dose interval at steady state were observed after ordinary capsules compared to controlled-release tablets. The extent of bioavailability was the same. Eight patients reported some side-effects during the capsule period and nine during the controlled-release tablet period.