To define the appropriate regime for the transition from intravenous lignocaine to oral tocainide after uncomplicated acute myocardial infarction, 43 patients received lignocaine to steady state. Each patient then received a tocainide dosage schedule. Plasma concentration of lignocaine and tocainide was measured frequently until the third peak plasma tocainide level. Tocainide 400 mg 8 hourly starting 4 h before cessation of lignocaine and tocainide 400 mg 4 hourly starting at the end of the infusion produced therapeutic plasma tocainide concentration (3.5-9 mg/l) only after the second dose. Tocainide 600 mg 12 hourly starting 6 h before cessation of lignocaine and tocainide 600 mg 6 hourly starting at the end of the infusion quickly achieved therapeutic plasma tocainide concentration which declined to give subtherapeutic first dose troughs of 2.42 mg/l (+/- 0.28 SEM) and 2.79 mg/l (+/- 0.27 SEM) respectively. Consistently therapeutic plasma tocainide concentrations were achieved by both of these regimens after the second dose. The short plasma half-life of lignocaine which for these regimes was 3.71 h (+/- 0.25 SEM), resulted in subtherapeutic lignocaine concentrations before consistently therapeutic plasma tocainide concentrations had been achieved. On the basis of these results, the 600 mg 6 hourly tocainide dosage schedule was studied with cessation of lignocaine infusion either two or six h after the first tocainide dose. With the former regime only three of 5 patients had therapeutic lignocaine at the subtherapeutic tocainide trough.(ABSTRACT TRUNCATED AT 250 WORDS)