Antiarrhythmic effectiveness of N-methyl-N-(beta-hydroxyethyl) guanidine O-phosphate (creatinol O-phosphate, COP) has been investigated in 10 patients with ischemic heart disease and frequent premature ventricular contractions (PVCs). Each patient received a random succession of treatment with the drug (1020 mg) and with a reference substance (solvent of COP) both administered i.m. twice a day over a 3-day period. In each patient a Holter ECG was recorded in the basal state and during the last 24 h of each treatment. Heart rate, PQ and QTc showed no changes. On the contrary, PVCs were significantly lower (P less than 0.01) with COP than with the reference substance. The drug showed a prevailing effect during daytime. Results are discussed in view of experimental observations suggesting that COP has a "membrane effect".