“…It has been reported previously that simultaneous administra tion of 5-hydroxytryptamine (5-IIT) with adrenaline reduced the toxicity of the latter compound on the heart of an animal anaes thetized with chloroform [2], During an investigation of the mechanism of this protective effect, it was felt that an increase in the coronary blood supply (which is usually produced by 5-IIT) might have prevented the onset of cardiac arrhythmia by belter oxygenation. This hypothesis has now been tested by determining the amount of chloroform required to produce an arrest of the isolated rabbit heart after induction of alterations in the coronary circulation by various drugs.…”