The antiarrhythmic action of GS 015 was studied in proportion to its plasma concentrations ascertained in parallel, making use of the model of the two-step coronary ligature in the conscious dog. Blood levels of 1.0 microgram/ml (2 mg/kg i.v.) or of 0.8 microgram/ml (5 mg/kg orally) brought about a complete suppression of ectopic arrhythmias. The limit concentration for this effect is about 0.5 microgram/ml.