In a double-blind investigation N-methyl-N-(beta-hydroxyethyl) guanidine O-phosphate (creatinol O-phosphate, COP) was checked versus a reference substance (solvent of COP) on volunteers affected by ischemic heart disease with persistent ventricular premature beats (VPB). COP was able to reduce VPB by 50--100% in 85% of the volunteers treated with this drug. This fact and the virtual absence of side-effects of COP lead the authors to the conclusion that COP merits more extensive investigations in this field in view of its clinical employment alone or in association with specific antiarrhythmic agents.