The antiarrhythmic action of GS 015 was studied proportionally to its plasma concentrations ascertained in parallel, using the model of the electrofibrillation of the cat's heart. Blood levels of about 1.3 micrograms/ml after i.v. injection of 2 mg/kg caused a marked short-term increase of the fibrillation threshold which then remained at the increased level for a longer period yet, observing GS 015 concentrations between 0.8 and 0.5 micrograms/ml. Individual differences existed in the height of the blood level as well as in its proportion to the effect.