Abstract: Electric field stimulation is widely used for heart pacing and arrhythmia reversion. In this study, we analysed the influence of waveform and direction of external stimulating electric field on the excitation threshold of isolated ventricular myocytes. The threshold field (ET) was lower when the field was applied longitudinally (ET,L) rather than transversally (ET,T) to the cell major axis. Rheobase was greater for transversal stimulation, but chronaxie and estimated membrane polarization were similar for both directions. The calculated maximal variation in membrane potential at the threshold (_VT ? 15 mV) was insensitive to field direction. As _VT values were similar, we assumed that the ET,T/ET,L ratio might be described solely as the ratio of the major and minor cell semi-axes. Accordingly, the ratio thus estimated was comparable to that determined experimentally. Stimulus waveform significantly affected both ET and _VT, which were greater for monophasic versus biphasic stimuli. Direction and waveform effects were independent. We conclude that (a) direction affects ET by its influence on the ability of a given field intensity to cause threshold membrane polarization and (b) thresholdlowering effects of longitudinal stimulation and biphasic waveforms apparently depend on different mechanisms, are additive and thus may be combined to decrease the energy requirement for myocardial stimulation Ver menos