1. Free plasma lignocaine concentrations were measured for up to 48 h after constant infusion of the drug in 41 subjects with suspected acute myocardial infarction. 2. The free plasma lignocaine clearance at 12 h was significantly and proportionately related to body weight and to the presence of mild (Killip Class II) heart failure, with an 18% reduction in free clearance in the latter condition. 3. The free plasma lignocaine was not related to sex, age or the presence of confirmed acute myocardial infarction, when corrected for the effects of body weight and presence of heart failure. 4. Free plasma lignocaine concentration 1 h after a fixed loading dose were also significantly related to body weight and presence of heart failure but not to sex, age or proven acute myocardial infarction. 5. The data indicate that correction of loading and maintenance infusion for body weight and presence of (even mild) heart failure should somewhat reduce variability in free (and presumably active) plasma lignocaine concentrations but that the free plasma lignocaine concentration at 12 h is most accurately predicted by measuring the free (and to a lesser extent total) plasma lignocaine concentration at 1 h.