N-Methyl-N-(beta-hydroxyethyl) guanidine O-phosphate (creatinol O-phosphate, COP), in previous pharmacological and clinical research showed useful effects on both normal and ischemic heart. These effects can be reconduced to a protective action of COP on cell membrane. In this investigation the possible electrophysiological effects of COP were ascertained in 6 patients, some of whom were free of excitation-conduction disturbances, while others were suffering from sinusal automaticity of A-V conduction alterations. When the nodal A-V conduction was slowed by iatrogenic or functional factors, COP as well as atropine improved it. Thus both COP and atropine act in the non-organic component of the conduction disturbances. The organic component was not affected by both the drugs. The effect of atropine was more evident than that of COP. All the electrophysiological parameters were not affected by COP. This fact allows COP to be administered with absolute safety even in the presence of alterations in the formation and conduction of the stimuli.