Procainamide (PA) pharmacokinetics were studied at steady state in 19 patients with myocardial infarction. PA was given 1.0 or 1.5 g/8 h (L [low] and H [high] dose groups) and acetylator phenotype determined from sulfadimidine acetylation was classified as slow or fast (S or F). Groups (4) were categorized, HF, LF, HS and LS with 7, 5, 3 and 4 patients in each respective group. Overall the mean steady state plasma concentration of PA (Cpss) expressed as a fraction of dose did not depend on dose or on acetylator status, but the HS group had significantly higher Cpss/g dose (6.3 .mu.g/ml) than LS (2.7 .mu.g/ml) or the HF (2.3 .mu.g/ml) groups. Clearance of PA by acetylation (ClA) was 23.8% of the total in fast acetylators and 16.5% in slow acetylators. ClA was not dose-dependent in the HF or LF groups (mean 177.9 ml/min and 168.4 ml/min) but was dose-dependent in the HS group (mean 74.6 ml/min) which differed significantly from the LS group (mean 113.4 ml/min).