The spontaneous ventricular fibrillation (VF) potential of the nonionic contrast media, ioversol (IOV), with and without the addition of sodium was examined during right coronary artery (RCA) injections into anesthetized closed-chest dogs. Protocols included fixed volume (6 mL) and fixed rate (0.4 and 0.6 mL/sec) injections to compare two or more of the following: IOV, IOV + (0.075-0.9% wt/vol) NaCl, and sodium/meglumine diatrizoate (DIA). In these studies, the incidence of VF for IOV alone was either greater that with IOV + NaCl formulations or, if equivalent, the incidence of other arrhythmias was greater with IOV alone than with the sodium formulations. When DIA was included in the comparisons, the incidence of VF was always greater than IOV with or without sodium. There was a sodium-related concentration prolongation in QT interval that, at 0.9% NaCl, approximated that with DIA, even though the incidence of VF for the sodium formulation was 0/15 vs. 6/12 for DIA. Thus, the addition of sodium to IOV appears to reduce the propensity for sponteneous VF in the canine model.