Various combinations of 1) cardiac preload, 2) sympathetic outflow to the heart, and 3) parasympathetic outflow to the heart may yield the same heart rate, while the resulting ventricular electrophysiological properties differ. This may be an important source of intra-individual electrocardiogram (ECG) variability. Isometric stress, by invoking the ergoreflex, increases sympathetic outflow. By an increased peripheral resistance this increases cardiac afterload, activating the arterial baroreflex. This reflex, in turn, reduces the sympatho-excitation and increases parasympathetic outflow. Progressive gravitational stress (sitting-standing) causes a progressively decreasing cardiac preload. Decreasing cardiac preload activates the cardiopulmonary baroreflex. This causes an increase of sympathetic outflow, while parasympathetic outlow changes in opposite direction. An example of intra-individual ECG variability in the sitting position is given for healthy men. Two stressors were applied: handgrip (ergoreflex) with the legs in horizontal position, and progressive leg lowering (gravitational load). Thirteen subjects had a heart rate match better than 1%. QRS azimuth, QRS duration, maximal T vector, T azimuth, T elevation, ST duration, QRS-T angle and QT interval differed significantly between handgrip and leg lowering (QT interval 435 +/- 21 vs. 418 +/- 15 ms, P <.01). More extreme experimental conditions and/or more sensitive substrates than healthy young men are likely to entail larger ECG differences. It may be clinically relevant to further explore this source of intra-individual ECG variability.