In order to compare the effectiveness of magnesium with that of proajmaline in preventing of recurrences of paroxysmal tachyarrhythmias. 60 consecutive patients were studied who had been previously treated for paroxysmal arterial fibrillation (PAF) or paroxysmal supraventricular tachycardia (PSVT). For the studies the patients were qualified who had been effectively treated during acute phase with intravenous magnesium (magnesium sulphate) and ajmaline (gilurytmal). The patients were randomly divided into three groups of 20 persons in each, every group received different oral treatment: in the first group--magnesium (magnesium carbonate) 3 x 0.6, in the second--proajmaline (neogilurytmal) in dose 3 x 20 mg, and in the third--neogilurytmal 2 x 20 mg and magnesium 2 x 0.6. During three months of observation the effectiveness of treatment and incidence of adverse effects were assessed. The following was found: neogilurytmal showed higher effectiveness (60%) than magnesium (30%), while the combined treatment demonstrated similar effectiveness (65%) as neogilurytmal alone with lower drug doses and lower incidence of bad tolerance manifestations (10% vs 20%).