A quantitative and qualitative analysis of ventricular arrhythmia was performed in 120 patients (64 men and 56 women, mean age 54 +/- 16) who suffered from arterial hypertension or congestive heart failure in the course of organic heart disease or ischaemic heart disease. 60 of them were treated with diuretics and the other 60 were control group. Neither antiarrhythmic drugs nor digitalis were used. There were no signs of left ventricular hypertrophy. Most patients treated with diuretics received potassium supplementation. Besides clinical examination all patients underwent 24 hours monitoring of Holter ECG. 38 patients treated with diuretics were evaluated before and after 6 months of therapy. In the diuretic group significantly higher percentage of patients with greater density of premature ventricular beats (count of premature ventricular beats [PVB]/100,000 heart evolutions) was observed. Number of patients with complex ventricular arrhythmia (Lown IVa and IVb) was also greater in this group. Serum levels of potassium and magnesium fell within the normal range, but the latter was significantly lower (p < 0.05) in those treated with diuretics.