Ventricular fibrillation threshold (VFT) was measured in anesthetized dogs, while cardiac work was altered by changing arterial pressure (pressure work) or by opening an aorta to left atrial shunt (flow work). When VFT during pressure work was compared with that during flow work, at a constant heart rate, it was found that there was a negative correlation (a lower threshold for fibrillation) between VFT and flow work but not pressure work. VFT also had negative correlations with the rate of contraction and relaxation indexes; maximal +/- dp/dt and negative dp/dt were the most significant variables. On the other hand, coronary flow, oxygen consumption, end diastolic pressure, and tension time index were not correlated with VFTs. Thus, VFT depended on mechanical factors not necessarily correlated to alterations in myocardial perfusion.