Intravenous magnesium is an effective treatment for ventricular tachycardia of some etiologies, and in patients with congestive heart failure low serum magnesium concentrations are associated with frequent arrhythmias and high mortality. This suggests that magnesium administration may decrease the frequency of ventricular arrhythmias in patients with heart failure. We therefore assessed the impact of an intravenous magnesium infusion upon the frequency of ventricular premature depolarizations in 40 patients with New York Heart Association (NYHA) class II to IV heart failure and serum magnesium < or = 2.0 mg/dl. Within 1 week of a baseline 6-hour ambulatory electrocardiographic recording, an infusion of 0.2 mEq/kg of MgSO4 was given over 1 hour and a repeat 6-hour recording was obtained. There was an inverse relationship between the change in magnesium concentration and the change in frequency of premature ventricular depolarizations; premature ventricular depolarizations declined by 134 +/- 207 hr-1 in patients in whom serum magnesium concentration increased > or = 0.75 mg/dl, but increased by 72 +/- 393 hr-1 in patients with a change < 0.75 mg/dl (p < 0.05). For all patients, the frequency of premature ventricular depolarizations was 283 +/- 340 hr-1 pretreatment and 220 +/- 269 hr-1 following magnesium infusion (p = 0.21). Patients with > or = 300 premature ventricular depolarizations hr-1 demonstrated a decrease from 794 +/- 309 to 369 +/- 223 hr-1 (p < 0.001). Intravenous magnesium administration decreased the frequency of couplets from 233 +/- 505 to 84 +/- 140 (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)