Ventricular dimensions by surface echocardiography and intraventricular pressures were monitored in 27 dogs before and during ventricular fibrillation (VF) induced by coronary embolization (nine dogs), potassium infusion (nine dogs) and calcium infusion (nine dogs). Left ventricular diameter (LVD) fell by an average of 10.3 mm during the first 30 s after the onset of VF induced by ischemia or potassium and remained smaller than the prefibrillation end-diastolic LVD during the ensuing 10 min. LVD fell during calcium infusion, and after the onset of VF it remained only slightly larger than the preinfusion end-systolic LVD. Right ventricular (RV) diameter increased progressively for the first 2 min during VF an average of 15.9 mm. The failure of LV size to increase during VF was explained by a pressure gradient inhibiting LV filling during the early phase of VF. Despite progressive RV filling, pressure in the more compliant RV remained lower than in the LV, which exhibited reduced compliance during VF. Therefore, cardiac dilation during VF appears to be confined to the RV, and inhibition to LV filling is an important feature of the syndrome.