OBJECTIVE The aim was to investigate the influence of the sympathetic nervous system on the induction of mechanical and electrical alternans in the intact canine heart. METHODS Experiments were performed on 8 open-chest dogs anesthetized with sodium pentobarbital. A micromanometer-tipped catheter was used to measure left ventricular pressure, dp/dt and the time constant of isovolumic relaxation. Rapid atrial pacing was used to induce alternans and the left stellate ganglion was stimulated electrically to alter sympathetic tone. The longest pacing cycle length that showed a significant alternation in peak systolic pressure was defined as the alternans threshold. Electrical alternans was detected by comparing the ST-T area in the surface ECG (lead II) on alternate beats. RESULTS The alternans threshold was 305(s.e.m. 10.4) ms under control conditions and decreased to 271(12.1), 225(33.4), and 177(6.2)ms, as the frequency of left stellate stimulation was increased to 1, 2, and 5 Hz, respectively (P < 0.001). Tau and peak -dp/dt began to alternate at the same pacing cycle length as peak +dp/dt and peak systolic pressure. Electrical alternans was only observed during mechanical alternans and the ST-T area of the strong beat was 243(143)% greater than the ST-T area of the weak beat (P < 0.001). Timolol (1 mg.kg-1) blocked the effect of left stellate stimulation (1 and 2 Hz) on mechanical and electrical alternans. CONCLUSIONS Left sympathetic activation causes a frequency-dependent reduction in the threshold cycle length for global mechanical and electrical alternans. Alternation in relaxation occurs at the same pacing cycle length as does alternation in contraction. Repolarization alternans in the surface ECG appears to reflect underlying mechanical events.