Sixty-four dogs were randomly assigned to receive either thiopentone or propofol and their electrocardiograms were recorded immediately before and shortly after they were anaesthetised. Thiopentone caused a marked increase in qt and jt intervals, a flattening of the T-wave and an increase in precordial qt dispersion. Propofol induced a less marked increase in qt and jt intervals, corrected for heart rate. Both agents induced an increase in heart rate and a decrease in heart rate variability, consistent with reduced vagal tone. Shortly after anaesthesia was induced, thiopentone affected ventricular repolarisation to a far greater extent than propofol, changes which suggest that it may be more likely to induce re-entrant ventricular arrhythmogenesis and could be associated with an increase in sympathetic tone. Propofol may therefore be more suitable than thiopentone for dogs with a susceptibility to ventricular arrhythmias or a long qt interval.