The effects of xylazine or medetomidine on tiletamine/zolazepam (Zoletile) anesthesia were evaluated by changes in the canine electroencephalogram (EEG). Experimental groups were three: the group treated with 10 mg/kg of Zoletile intravenously (TZ), the group treated with 1.1 mg/kg of xylazine intramuscularly and 10 mg/kg of Zoletile intravenously (XTZ), and the group treated with 30 microg/kg of medetomidine intramuscularly and 10 mg/kg of Zoletile intravenously (MTZ). EEG recording electrode was positioned at Cz, which was applied to International 10-20 system. For all recording times, the powers of each band (band 1: 1-2.5 Hz, band 2: 2.5-4.5 Hz, band 3: 4.5-8 Hz, band 4: 8-13 Hz, band 5: 13-20 Hz, band 6: 20-30 Hz, band 7: 30-50 Hz, band 8: 1-50 Hz) were calculated. In TZ, at 10 min after Zoletile injection, the powers of bands 3, 4, 5 and 8 significantly decreased. At 20 min after Zoletile injection, the powers of band 1 and 8 were significantly decreased. After Zoletile injection, there were significant decreases in bands 1, 4, 5, 6, 7 and 8 in XTZ, and in bands 1, 3, 4, 5, 6, 7 and 8 in MTZ. These significant changes disappeared in all band powers in TZ and MTZ in the last 1 min just prior to the dogs showing head-up movement. But, in XTZ, all band powers, except band 2, were significantly reduced. EEG power spectral analysis revealed the differences in band powers on awakening, even though the same kind of drugs were used. We thought that was a useful method to compare the effect of xylazine and medetomidine on Zoletile-induced anesthesia in dogs.