The kinetics of lidocaine after i.m. injection (M. deltoideus) of 300 mg were studied in patients with acute myocardial infarction in the course of 4 hrs.; another patient received this i.m. dose after being pretreated with 100 mg lidocaine i.v. The antiarrhythmically effective threshold concentration in plasma (1.5 mg/l) was reached after an average time of 5.5 min by 9 out of 10 patients. The mean value of maximum concentration after 30 min was 3.6 mg/l in 10 patients and the mean duration for exceeding the threshold (1.5 mg/l) was about 2 hrs. The elimination half-life for lidocaine valued 1.77 hrs. The toxic threshold concentration of 6 mg/l was exceeded for a short time by 3 patients without any side effects; in three other patients suffering from side-effects no relation to high plasma levels of lidocaine was observed.