This study aimed to compare four protocols for preanesthetic insulin administration and fasting time with respect to the variation of intraoperative blood glucose (BG) concentrations versus preanesthetic values (baseline). The patient records of dogs undergoing cataract surgery were included. Data on anesthetic protocols, comorbidities, and intraoperative complications (hyper- and hypoglycemia, hypotension, hypothermia, and bradycardia) were analyzed. The insulin/fasting protocols included (A) 12 hr fasting and half insulin dose, (B) 6 hr fasting and half insulin dose, (C) 12 hr fasting and full insulin dose, and (D) 12 hr fasting and no insulin. Forty-eight dogs were included (14 in A, 10 in B, 13 in C, and 11 in D). Protocol D resulted in a significant increase of intraoperative BG concentrations compared with baseline (P = .001), whereas in the remaining groups, the baseline BG did not differ from intraoperative values. There were no statistically significant associations between the treatment group and the occurrence of intraoperative complications or the presence of diagnosed comorbidities. In conclusion, different insulin and fasting regimen protocols may be used for diabetic patients with no apparent benefit or risk from one protocol versus another. The use of insulin before surgery results in lesser increase of BG intraoperatively as compared with preanesthetic values. However, whether this should be interpreted as better perioperative control of glycemia remains debatable.