Propylene glycol and benzyl alcohol, the main constituents of most solvent vehicles, display a pronounced antiarrhythmic-antifibrillatory effects, when injected intravenously into animals (dogs, rats) with spontaneous or drug-induced arrhythmias. The antiarrhythmic dose for propylene glycol amounts to 0.2-0.3 ml/kg of a 70 per cent solution and, for benzyl alcohol to 0.2-0.4 ml/kg of a 4 per cent solution in physiologic saline, respectively. Similar effects were also obtained by the combined injection of propylene glycol + benzyl alcohol, in proportions which correspond to the formulae of numerous commercial "solvents" (vehicles): 2 to 20 per cent solutions of benzyl alcohol in 70 per cent propylene glycol (0.05-0.2 ml/kg). The mechanisms which might be responsible for the antiarrhythmic activity of solvents are discussed: lengthening of the effective refractory period, local and general anaesthetic effects, changes of osmolarity. The intravenous injection of propylene glycol and/or benzyl alcohol, in high doses, produces intravascular haemolysis. Clinical investigations are recommended as to the potential, beneficial or toxic effects of drug solvents, especially upon the cardiocirculatory system.