The auditory evoked potential (AEP) is correlated to anaesthetic depth. The AEP has been used in rats, pigs, dogs and humans to assess anaesthetic depth. This study was undertaken to determine whether the AAI Index® derived from the AEP correlated with changes in end tidal isoflurane concentration in dogs. The average AAI Index was 21.8 ± 10.5 and isoflurane concentration was 1.7±0.4 %. Data were divided into 0.5 % intervals of end tidal anaesthetic agent concentration (ETAA). When ETAA values were higher than 2.5 % the AAI values were 2.1-2.5 %, 1.6-2.0 % and 1.1-1.5 % higher than AAI values although not statistically different. The 2.1-2.5 % interval was statistically different from the 1.1-1.5 % and <1.1 % interval. The 1.6-2.0 % interval was statistically different from the 1.1-1.5 % and the <1.1 % intervals. The 1.1-1.5 % interval was statistically different from the <1.1 % interval. The correlation between the AAI Index and isoflurane was -0.176 and was statistically significant (P=0.0009). A linear regression between the AAI Index and isoflurane revealed the following relationship: AAI = 29.074 - (4.2755 × isoflurane) with a power of 0.913. The polynomial regression relationship was AAI = 53.334 - (35.715 x isoflurane) + (10.322 x isoflurane2) - (0.43646 x isoflurane3) with a power of 0.999. The AAI Index was found to correlate with changes in isoflurane concentration.